Methicillin-susceptible S. Aureus Strains Research Articles

Detection of methicillin-resistant Staphylococcus aureus (MRSA) carrying the mecC gene in wild small mammals in Spain

To determine the rate of Staphylococcus aureus faecal carriage in 101 wild small mammals in Spain and to characterize the isolates obtained. Faecal samples were seeded on mannitol salt agar and ORSAB plates. The presence of the resistance genes mecA, mecC and blaZ and the new blaZ allotype associated with staphylococcal cassette chromosome mec (SCCmec) XI (blaZ-SCCmecXI) was studied by PCR. S. aureus isolates were characterized by spa typing, agr typing and multilocus sequence typing. The presence of immune evasion cluster (IEC) genes and virulence genes was analysed by PCR. S. aureus was detected in 13/101 studied faecal samples and one isolate per positive sample was further studied. Two S. aureus isolates were methicillin-resistant S. aureus (MRSA) (recovered from wood mice, Apodemus sylvaticus) and 11 were methicillin-susceptible S. aureus (MSSA). Both MRSA isolates harboured the mecC gene and the novel blaZ-SCCmecXI, were typed as spa-t1535/agrIII/ST1945(CC130)/SCCmecXI (where ST stands for sequence type and CC stands for clonal complex), carried the exfoliative toxin etd2 gene and were IEC type E. Eight different spa types were identified among the 11 MSSA isolates (five new) and six different sequence types were identified (two new). All MSSA strains were susceptible to the antibiotics tested except one blaZ-positive penicillin-resistant isolate (spa-t120/agrII/ST15). MSSA isolates were ascribed to the CCs (number of strains) CC5 (1), CC1956 (4) and singleton (6). Nine of 11 MSSA isolates carried the cna virulence gene. Only one MSSA isolate carried IEC genes (type C). This is the first report of MRSA carrying mecC in faecal samples of wild small mammals in Spain. These resistant isolates carried genes of the IEC system, unusual in S. aureus from animals. Wild small mammals could be a reservoir of the mecC gene with important implications for public health.

Genetic Lineages, Antimicrobial Resistance, and Virulence in Staphylococcus aureus of Meat Samples in Spain: Analysis of Immune Evasion Cluster (IEC) Genes

The objective of this study was to determine the rate of contamination by Staphylococcus aureus in 100 meat samples obtained during 2011-2012 in La Rioja (Northern Spain), to analyze their content in antimicrobial resistance and virulence genes, as well as in immune evasion cluster (IEC) genes, and to type recovered isolates. Seven of 100 samples (7%) contained S. aureus: 6 samples harbored methicillin-susceptible S. aureus (MSSA) and 1 pork sample harbored methicillin-resistant S. aureus (MRSA). The MRSA isolate corresponded to the ST398 genetic lineage with a multidrug resistance profile and the absence of human IEC genes, which pointed to a typical livestock-associated MRSA profile. MRSA isolate was ascribed to the spa-type t011, agr-type I, and SCCmec-V and showed resistance to erythromycin, clindamycin, tetracycline, and streptomycin, in addition to β-lactams. The remaining six MSSA strains belonged to different sequence types and clonal complexes (three isolates ST45/CC45, one ST617/CC45, one ST5/CC5, and one ST109/CC9), being susceptible to most antibiotics tested but showing a wide virulence gene profile. Five of the six MSSA strains (except ST617/CC45) contained the enterotoxin egc-cluster or egc-like-cluster genes, and strain ST109/CC9 contained eta gene (encoding exfoliatin A). The presence of human IEC genes in MSSA strains (types B and D) points to a possible contamination of meat samples from an undefined human source. The presence of S. aureus with enterotoxin genes and MRSA in food samples might have implications in public health. The IEC system could be a good marker to follow the S. aureus contamination source in meat food products.

Genotypic Characterization of Staphylococcus aureus Isolates From Cases of Keratitis and Healthy Conjunctival Sacs

Staphylococcus aureus is the predominant pathogen that causes keratitis, and the rate of occurrence of drug-resistant S. aureus is increasing. However, little is known about its clinical epidemiology in Japan. This study was designed to characterize the genotypes of methicillin-susceptible S. aureus (MSSA) and methicillin-resistant S. aureus (MRSA) isolates from keratitis and healthy conjunctival sacs. Thirty keratitis isolates (19 MSSA and 11 MRSA) and 18 healthy conjunctival sac isolates (16 MSSA and 2 MRSA) obtained before cataract surgery were used. They were characterized by multilocus sequence typing. The prevalence of Panton-Valentine leukocidin was determined. In multilocus sequence typing analysis, ST5 and ST764 (ST5 variant) were the most frequently identified sequence types in MRSA strains from both keratitis and healthy conjunctival sacs. ST188 was the most frequently identified sequence type in the 19 MSSA isolates from keratitis (4 isolates, 21.1%), but was only identified in 1 of the 16 commensal MSSA strains. ST8 was found to be predominant among the 16 commensal MSSA strains (5 isolates, 31.3%). None of the isolates had Panton-Valentine leukocidin genes. MRSA isolates from keratitis and healthy conjunctival sacs may have similar genotypic characteristics, but certain clones occur more often among MSSA isolates from keratitis than among commensal MSSA strains. These results suggest that specific MSSA lineages that possess genotypic characteristics can more effectively cause keratitis.

Staphylococcus aureus ST398 from slaughter pigs in northeast China

To describe the prevalence and population structure of Staphylococcus aureus bacteria that colonize pigs at slaughterhouses in northeastern China, nose swabs were collected from pigs in two slaughterhouses in Harbin, Heilongjiang Province, China in 2009. S. aureus isolates were characterized by multilocus sequence typing (MLST), spa typing, SCCmec typing, antimicrobial susceptibility testing and pvl gene detection. A total of 200 S. aureus isolates were collected from 590 pigs (33.9%, 200/590), of which 162 (81%, 162/200) were methicillin-susceptible S. aureus (MSSA) and 38 (19%, 38/200) were methicillin-resistant S. aureus (MRSA). Ninety-nine of the MSSA isolates (99/162, 61.1%) were ST398, which represented the dominant sequence type overall. Eighty-seven isolates were ST9 (87/200, 43.5%), and all MRSA belonged to that sequence type which consisted of the spa types t899 and t2922. Among the MSSA strains, t034, t899 and t4358 were the most dominant spa types (139/162, 85.8%). All MRSA isolates harbored SCCmec type IVb. The pvl gene was only detected in 3 ST7/t2119 MSSA isolates. All MRSA but more importantly also 82.7% (134/162) of the MSSA isolates were resistant to six or more antibiotics. Moreover, a novel resistance determinant-lsa(E) was identified among 22% (44/200) of all isolates. In conclusion, pigs in northeast China are frequently colonized with ST398 MSSA. MRSA with this sequence type, typically associated with pigs in Europe, was not found. High levels of multiple antibiotic resistance among MRSA isolates as well as MSSA isolates are a public health concern.

Molecular Diagnosis of Methicillin-Resistant Staphylococcus aureus Colonization

Trouillet-Assant et al. (1) described the difficulties in developing a molecular methicillin-resistant Staphylococcus aureus (MRSA) screening assay. Cocolonization with methicillin-susceptible S. aureus (MSSA) and methicillin-resistant coagulase-negative staphylococcus (MRCoNS) led to false-positive results in thefirst-generation assays (multiplex assays targeting the mecA gene and a S. aureusspecific gene). Likewise, drop-out mutants contributed to falsepositive results in the second-generation assays (multiplex assays that target orfX-staphylococcal cassette chromosome mec element [SCCmec] junction sequences). For this reason, third-generation assays were developed (multiplex assays that target mecA/mecC genes, orfX-SCCmec junction sequences, and a S. aureus-specific gene). A false-positive reaction can still occur in the presence of a MRCoNS strain but only if the MSSA strain is a drop-out mutant. The frequencyofdrop-outmutantsisvariable,reportedfrom4.5%(2)toas high as 12.5% (3). Recognizing that the prevalence of drop-out mutants can be influenced by clonal spread, it would still be useful for Trouillet-Assantetal.toreportthefrequencyofdrop-outmutantsin their patient populations, as well as the frequency of false-positive molecular assays caused by the presence of MRCoNS.

Understanding the Molecular Pathogenesis of Methicillin-Resistant Staphylococcus aureus

Staphylococcus aureus has been referred to as the “persistent pathogen” [1, 2] owing to its high prevalence for decades, its cause of a significant burden of disease, and its ability to cause a wide spectrum of clinical infections at various sites, including skin and soft tissue, bone and joint, heart valves, blood stream, and cerebrospinal fluid. These organisms possess numerous toxins and virulence factors that may account for their virulence and ability to evade host defenses [3, 4]. The acquisition by S. aureus of the staphylococcal cassette chromosome containing the mecA gene (SCCmecA), which encodes for methicillin resistance, has added to its armamentarium and increased the difficulty in its eradication in human infections [5,6]. The epidemiology of methicillin-resistant S. aureus (MRSA) is closely intertwined with that of methicillin-susceptible S. aureus (MSSA), and has become more complex with the recognition of healthcare-associated, healthcare-associated community-onset, and community-associated (CA) MRSA strains [4, 7, 8]. With the advent of numerous molecular typing methods has come an enhanced understanding of the complex evolution and epidemiology of MRSA [5, 9, 10]. There is a predominance of multilocus sequence type ST5 or clonal complex (CC) 5, with pulsed-field gel electrophoresis (PFGE) type USA100 as the most common healthcare-associated strains and ST8 or CC8 PFGE USA300 as the most common among communityassociated strains [8, 9]. Many of the MRSA strains are thought to have originated from predominant MSSA strains that acquired SCCmecA [6, 11]. The genotype classifications, while critical in elucidating the evolution, transmission, and epidemiology of both MSSA and MRSA infections in populations at risk, have only recently been recognized as potential predictors of specific clinical infections. As examples, in a large international data set of MSSA isolates, isolates causing infective endocarditis were more likely to be CC33 and to contain various adhesins and enterotoxins [12]. Furthermore, an analysis of both MSSA and MRSA isolates from one US academic medical center demonstrated that CC5 and CC30 strains were responsible for hematogenous complications of infection [13]. A predominant Asian strain of MRSA, ST239, was recently responsible for a large intensive care unit outbreak in London, and was highly associated with vascular access device-related bacteremia [14]. A related predominant Brazilian clonal complex of MRSA has been demonstrated to have enhanced ability to produce biofilm and adhere to and invade airway epithelial cells, properties that may promote its nosocomial transmission and ability to cause pneumonia [15]. In a study of patients with S. aureus bacteremia, CC15 was associated with 30-day mortality, and CC22 with osteoarticular infections [16]. These and other findings suggest that clonal typing may be a potential predictor of specific clinical infections. Additional research is needed to analyze the specific features and characteristics of individual clonal types that could be responsible for their predilection to cause or associate with specific types of clinical infections. One very important area of future research is linking the profiles of S. aureus toxins and virulence factors to specific genotypes. Because of their physiologic functions, specific toxins and virulence factors are probably the key to and the best predictors of important infection [3]. One of the most commonly cited factors in MRSA infections has been the PantonValentine leukocidin (PVL), a toxin [4, 17] that is found in many strains of CAMRSA and is cytotoxic to human and rabbit monocytes and macrophages and to human polymorphonuclear leukocytes. Received 11 October 2013; accepted 15 October 2013; electronically published 26 October 2013. Correspondence: Kurt B. Stevenson, MD, MPH, Division of Infectious Diseases, Department of Internal Medicine, College of Medicine, Ohio State University, N-1122 Doan Hall, 410 West 10th Ave, Columbus, OH 43210 (kurt.stevenson@osumc.edu). The Journal of Infectious Diseases 2014;209:488–90 © The Author 2013. Published by Oxford University Press on behalf of the Infectious Diseases Society of America. All rights reserved. For Permissions, please e-mail: journals. permissions@oup.com. DOI: 10.1093/infdis/jit557

Dynamic pattern and genotypic diversity of Staphylococcus aureus nasopharyngeal carriage in healthy pre-school children

It is common wisdom that persistent carriage of Staphylococcus aureus is more frequent in young children than in adults. The objectives of this study were to assess the S. aureus temporal carriage pattern among a healthy community of pre-school children, with concomitant description of genotype diversity, toxin-encoding genes and antibiotic resistance. Among 333 children 3-6 years of age, S. aureus nasopharyngeal carriage was assessed over one school year by culture of three sequential nasopharyngeal aspirates. Identification, methicillin resistance and toxin production profile were determined by PCR. Genotyping was performed by spa sequencing and multilocus sequence typing (MLST). Out of 830 samples collected, 286 (34%) yielded S. aureus from 185 carriers (55%). Based on consecutive genotype analysis, only 40/268 (15%) children could be classified as persistent carriers, and the remaining 118 (44%) showed intermittent carriage. spa typing revealed 82 types clustered into 13 spa clonal complexes (CCs). Fourteen strains isolated from 11 (3%) children were methicillin-resistant S. aureus (MRSA), half of these strains belonged to the commonly hospital-associated spa t008-ST8-SCCmec IV. Methicillin-susceptible S. aureus (MSSA) were genotypically more diverse. Toxic shock syndrome toxin and egc1/2 complexes were highly prevalent (24%). Contrastingly, Panton-Valentine leucocidin (PVL) was carried only by three MSSA strains (0.6% of children). Exfoliative toxins were detected in 10 (3.5%) MSSA strains, of which 5 were related to the impetigo clone CC121. Although S. aureus nasopharyngeal carriage was high among healthy pre-school children, persistent carriage seems to be less frequent than previously reported. The prevalence of MRSA carriage was 3%, but was not associated with PVL.

Phenotypic and Genotypic Characterization of Methicillin-Resistant Staphylococcus aureus (MRSA) and Methicillin-Susceptible Staphylococcus aureus (MSSA) from Different Sources in China

A diverse collection of 261 Staphylococcus aureus strains from human, animal, food, and environmental sources were tested for the presence and type of SCCmec elements, antibiotic susceptibility to various antibiotics, and non-ß-lactam antibiotic resistance genes. About 18.39% (48/261) of strains were methicillin-resistant S. aureus (MRSA) including 29.75% (36/121) human strains of which 29 strains were hospital-acquired MRSA (HA-MRSA) and 7 strains were community-associated MRSA (CA-MRSA) and 19.67% (12/61) animal strains that all were CA-MRSA strains. The percentage of CA-MRSA strains from animals was significantly higher than that from human (p<0.01). Most of MRSA strains and a part of methicillin-susceptible S. aureus (MSSA) strains harbored unique combinations of non-ß-lactamase genes aac(6')/aph(2″), aph(3')-III, ant (4',4″), ermA, ermC, mrsA, tetM, and tetK. Antibiotic resistance genes were detected more frequently in HA-MRSA strains than in CA-MRSA strains (p<0.01). MRSA strains and MSSA strains had 22 and 39 antibiotic profiles to 15 tested antibiotics, respectively. The resistant proportion was higher in HA-MRSA strains than in CA-MSSA strains for various antibiotics, as well as higher in MRSA strains than in MSSA strains. Animal MRSA reservoirs (particularly pigs and cows) might represent an important source of human CA-MRSA. CA-MRSA strains might acquire more different resistance genes gradually, depending on the selective pressure of antibiotics in different regions or environments. CA-MRSA is not yet endemic in China, but could be prevalent in future, contributing to its acquiring more resistance genes and huge animal sources. Infection with multidrug-resistant MSSA strains acquired from food, animal, and human sources might also become a significant problem for human medicine, which warrants further study.

Thioridazine potentiates the effect of a beta-lactam antibiotic against Staphylococcus aureus independently of mecA expression

The neuroleptic antipsychotic derivate thioridazine has been shown to increase the susceptibility of a methicillin-resistant Staphylococcus aureus (MRSA) isolate towards dicloxacillin. The aim of this study was to investigate the combinatorial effect of the two drugs on a broad selection of staphylococcal strains by analyzing a large collection of MRSA strains carrying different types of SCCmec, as well as MSSA strains. Transcription and translation of the resistance marker PBP2a encoded by mecA within the SCCmec cassette were analyzed by primer extension and western blotting.We observed increased susceptibility to dicloxacillin in the presence of thioridazine in all tested MRSA isolates. In contrast to previously published results, the synergistic effect was also applicable to methicillin-susceptible S. aureus (MSSA). We conclude that the combination of dicloxacillin and thioridazine potentiates the killing effect against S. aureus in a broad selection of clinical isolates. Additionally, the study indicates that the killing effect by the combinatorial treatment is independent of PBP2a-mediated resistance mechanisms.

Genetic environment and location of the lnu(A) and lnu(B) genes in methicillin-resistant Staphylococcus aureus and other staphylococci of animal and human origin

To detect the presence of lnu genes in staphylococcal strains with the unusual phenotype lincosamide resistance/macrolide susceptibility (L(R)/M(S)), and to determine their locations and genetic environments. Six staphylococcal strains of human and animal origin with the phenotype L(R)/M(S) were studied. The presence of 15 resistance genes was tested by PCR. SCCmec typing was performed for all methicillin-resistant strains. agr typing, spa typing and multilocus sequence typing were carried out for Staphylococcus aureus strains. Transformation experiments were carried out by electrotransformation. Plasmid or chromosomal gene location was determined by Southern blot analysis and the genetic environments of the lnu genes were studied in all strains. Three methicillin-resistant staphylococcal strains contained the lnu(A) gene. The presence of the pLNU1 plasmid carrying lnu(A) was confirmed in one methicillin-resistant S. aureus (MRSA) ST398-t108 and one methicillin-resistant Staphylococcus sciuri. A novel lnu(A)-carrying plasmid (pUR5425) was identified in one MRSA ST125-t067 strain. Transformants of the three lnu(A)-positive strains presented increased lincomycin MIC values. The remaining three studied staphylococcal strains harboured the lnu(B) gene: two methicillin-susceptible S. aureus (MSSA) ST9-t337 and one MRSA ST398-t011. The lnu(B) gene was embedded in the chromosome in the two MSSA strains and in a large-sized plasmid in the MRSA strain. The same lnu(B) genetic environment was detected in these three strains. The resistance phenotype L(R)/M(S) seems to be related to S. aureus animal-associated clonal lineages (ST398 and ST9). A novel lnu(A)-carrying plasmid was identified and this is the first detection of the lnu(B) gene in MRSA ST398.

Characterization of cell membrane parameters of clinical isolates of Staphylococcus aureus with varied susceptibility to alpha-melanocyte stimulating hormone

Staphylococcus aureus (S. aureus), a major human pathogen of hospital and community acquired infections, is becoming resistant to almost all commercially available antibiotics. This has prompted development of antimicrobial peptides as therapeutic options. Alpha melanocyte stimulating hormone (α-MSH) is one such peptide known to possess antimicrobial properties. In the present study, we analyzed the antimicrobial activity of α-MSH against 75 clinical strains of S. aureus including both methicillin susceptible S. aureus (MSSA) and methicillin resistant S. aureus (MRSA) strains. Results of our previous study showed that membrane damage is the major mechanism of staphylocidal activity of α-MSH. In this context, we compared the various bacterial membrane parameters, viz., membrane fluidity, lipid composition, and surface charge of a few selected MSSA and MRSA strains that showed variable susceptibility to the melanocortin peptide. Our results showed that α-MSH killed both type of strains efficiently (≥70% killing in 84% clinical strains after exposure with 6μM of α-MSH for 1h). It was observed that compared to the α-MSH-susceptible strains, the α-MSH-non-susceptible strains had a different membrane order and phospholipid pattern. There was no consistent pattern of cell surface charge to distinguish α-MSH-susceptible strain from a non-susceptible strain. In conclusion, α-MSH possessed potential staphylocidal activity for both against MSSA and MRSA strains. S. aureus strains not susceptible to the peptide exhibited a rigid membrane and a higher amount of the cationic phospholipid as compared to the α-MSH-susceptible strains.

Comparison of Methicillin-Resistant and Methicillin-Sensitive Staphylococcus aureus Strains Isolated from a Tertiary Hospital in Terengganu, Malaysia

Staphylococcus aureus is a persistent human pathogen responsible for a variety of infections ranging from soft-tissue infections to bacteremia. The objective of this study was to determine genetic relatedness between methicillin-resistant S. aureus (MRSA) and methicillin-susceptible S. aureus (MSSA) strains. We isolated 35 MRSA and 21 MSSA strains from sporadic cases at the main tertiary hospital in Terengganu, Malaysia, screening them for the presence of virulence genes. Their genetic relatedness was determined by accessory gene regulator (agr) types, PCR-restriction fragment length polymorphism (RFLP) of the coa gene, pulsed-field gel electrophoresis (PFGE), S. aureus protein A (spa), and multilocus-sequence typing (MLST). We found that 57% of MRSA and 43% of MSSA strains harbored enterotoxin genes. The majority (87.5%) of the strains were agr type I. PCR-RFLP and PFGE genotyping of the coa gene revealed that MRSA strains were genetically related, whereas MSSA strains had higher heterogeneity. The combined genotype, MLST-spa type ST239-t037, was shared among MRSA and MSSA strains, indicating that MRSA strains could have evolved from MSSA strains. Two combined MLST-spa types were present in MRSA strains, whereas 7 different MLST-spa types were detected in MSSA strains, including 2 combined types (ST779-t878 and ST1179-t267) that have not been reported in Malaysia. In conclusion, enterotoxin genes were more prevalent in MRSA than in MSSA strains in the Terengganu hospital. The MSSA strains were genetically more diverse than the MRSA strains.

A Caenorhabditis elegans Host Model Correlates with Invasive Disease Caused by Staphylococcus aureus Recovered during an Outbreak in Neonatal Intensive Care

Caenorhabditis elegans has previously been used as a host model to determine the virulence of clinical methicillin-resistant Staphylococcus aureus isolates. In the present study, methicillin-susceptible S aureus (MSSA) strains associated with an outbreak in a neonatal intensive care unit (NICU) were investigated using the C elegans model. Two distinct outbreak clones, MSSA type-C and MSSA type-G, were identified by pulsed-field gel electrophoresis in a MSSA outbreak during a seven-month period in the NICU of the Sunnybrook Health Sciences Centre (Toronto, Ontario). MSSA type-C was associated with severe infection, while type-G was associated with less invasive disease. Four representative type-C isolates, three type-G and three infant-colonized isolates unrelated to the outbreak, were sent to Calgary (Alberta), for the double-blinded virulence tests in the C elegans host model and for further molecular characterization. The invasive outbreak strains (type-C) demonstrated highly nematocidal activity, the noninvasive outbreak strains (type-G) an intermediate virulence, and the outbreak-unrelated colonization isolates demonstrated avirulence or low virulence in the C elegans model, with mean killing rates of 93.0%, 61.0% and 14.4% by day 9, respectively, for these three group strains. Different group MSSA strains had their own unique genetic profiles and virulence gene profiles, but all isolates within the same group (type-C or type-G) shared identical genetic characteristics and virulence gene patterns. The present blinded evaluation demonstrated that the nematocidal activities of MSSA strains correlated well with the clinical manifestation in an MSSA outbreak in the NICU, supporting C elegans as a robust host model to study the pathogenesis of S aureus.

Staphylolysin is an effective therapeutic agent for Staphylococcus aureus experimental keratitis

Therapy of S. aureus keratitis is increasingly challenging due to emerging resistant strains. Staphylolysin (LasA protease) is a staphylolytic endopeptidase secreted by Pseudomonas aeruginosa. The purpose of the current study was to study the effect of treatment with staphylolysin on experimental keratitis caused by various Staphylococcus aureus strains. The therapeutic effect was studied in a keratitis model induced in rabbits by intrastromal injections of 10(3) S. aureus cells of three different methicillin-resistant S. aureus (MRSA) strains and one methicillin-susceptible S. aureus strain (MSSA). Topical treatment with either staphylolysin or bovine serum albumin (BSA; control) was applied every half hour for 5h, starting at 4h after infection. Corneas were removed for bacterial quantification. Histopathological analysis was performed on MSSA-infected rabbits, killed at either one or 84h after completion of treatment and on uninfected eyes 1h after treatment termination. The number of bacteria in the staphylolysin-treated corneas was significantly reduced in all infections with the four S. aureus strains studied as compared to controls: the staphylolysin-treated eyes infected with MRSA strains were either completely sterilized or showed a 3-4 orders of magnitude decrease in the number of cfu/cornea (p = 0.004 to 0.005); all of the staphylolysin-treated MSSA-infected eyes were sterile. Histopathological analysis of the methicillin-sensitive (MSSA) strain-infected eyes at 84h after completion of treatment showed moderate inflammation in the staphylolysin-treated eyes as compared with extensive abscess formation in the control group. The uninfected corneas showed only mild stromal edema in both the staphylolysin and BSA-treated groups. Staphylolysin provided long-lasting protection against several strains of S. aureus, evident by both its strong anti-bacterial activity and beneficial histopathological results of treatment.

Nasal carriage of methicillin resistant Staphylococcus aureus and their antibiotic susceptibility patterns in children attending day-care centers

Nasal colonization with community acquired methicillin resistant Staphylococcus aureus (CA-MRSA) is being increasingly reported, especially in places where people are in close contact and in reduced hygiene, such as day-care centers. In this study we investigated the frequency of MRSA colonization and their antibiotic susceptibility patterns in 1-6 years old children of day-care centers in Hamadan, West of Iran.Five hundred nasal swabs were collected from children of 27 day-care centers that had no risk factors for colonization by S. aureus. The specimens were cultured for isolation of S. aureus by standard methods. Antimicrobial susceptibility testing was performed according to the Clinical and Laboratory Standards Institute (CLSI) guidelines. For evaluation of the frequency of erythromycin induced clindamycin resistance, disk approximation test (D-test) was applied.Totally, 148 (29.6%) children were colonized by S. aureus. Out of 260 male, 94 (36.2%) and of 240 female, 54 (22.5%) cases were nasal carriers of S. aureus (P value = 0.001). Six (4.1%) of the 148 S. aureus isolated from children were MRSA strains. None of MRSA and methicillin susceptible S. aureus (MSSA) was resistant to vancomycin and clindamycin. Three of the 6 strains of MRSA and 7 (4.9%) of the 142 MSSA strains were resistant to erythromycin, and D-test was positive in all of them.We conclude that the rate of colonization by S. aureus is high in children attending day-care centers but colonization with MRSA is not common in our areas. Clindamycin or trimethoprim-sulfamethoxazol could be used in mild to moderataly severe diseases caused by CA-MRSA. However, if the CA-MRSA isolates are erythromycin resistant, D-test should be carried out for detection of inducible clindamycin resistance.

Virulence Factors and Genetic Characteristics of Methicillin-Resistant and -SusceptibleStaphylococcus aureusIsolates in Myanmar

Staphylococcus aureus produces virulence factors, including various exotoxins and adhesins, which are associated with a variety of symptoms caused by its infections. In the present study, the prevalence of these virulence factors was analyzed for 23 S. aureus strains isolated from wound infections in hospitals, nasal swabs, or vomit from patients and cooks in a food poisoning case and from healthy adults in Yangon, Myanmar. Among these strains, five were methicillin-resistant S. aureus (MRSA) derived from pus (four strains, SCCmec III, ST239) and a healthy adult (one strain, SCCmec-IVa, ST5). The Panton-Valentine leukocidine (PVL) gene was detected in five methicillin-susceptible S. aureus (MSSA) clinical strains belonging to ST121 (CC121). The MRSA clinical strains had only a few or no staphylococcal enterotoxin (SE) genes, whereas PVL-positive MSSA and an MRSA strain from a healthy adult possessed an enterotoxin gene cluster (seg, sei, sem, sen, seo, and selu). Strains from the food poisoning case had either SE genes or only etd and edin-B. Adhesin genes, which are associated with binding to fibronectin, fibrinogen, and elastin, were detected in all the MRSA and most of the MSSA strains examined. However, the bone sialoprotein-binding protein gene (bbp) and the variant form of the elastin-binding protein gene (ebpS-v) with an internal 180 bp deletion were identified only in the MSSA strains harboring the PVL gene. These findings suggest that those genetic traits are characteristic of PVL-positive ST121 S. aureus strains in Myanmar.

Lethal Necrotizing Pneumonia Caused by an ST398Staphylococcus aureusStrain

To the Editor: The prevalent colonization of livestock with methicillin-resistant Staphylococcus aureus (MRSA) sequence type (ST) 398 in many countries is a cause for consternation. However, understanding of the emergence of these organisms and their public health implications is embryonic. The perceptions that all MRSA found in livestock are of ST398 lineage or that livestock are the only reservoirs of ST398 oversimplify a complex epidemiology, therefore, prudence is required when attributing human infections with S. aureus ST398 to livestock reservoirs. The fatal infection of a young girl with ST398 methicillin-susceptible S. aureus (MSSA) is tragic (1). However, the conclusion by the authors that “the spread of S. aureus ST398 among livestock is a matter of increasing concern because strains of this sequence type were able to acquire PVL [Panton-Valentine leukocidin] genes” is misleading. The authors report no history of livestock exposure and the spa type reported (t571) is relatively rare among livestock isolates (2,3). The isolate from the fatal case was tetracycline-susceptible and positive for PVL toxin, while livestock ST398 isolates have been almost uniformly tetracycline resistant and PVL negative. Notably, spa type t571 ST398 MSSA was detected in 9 families from the Dominican Republic living in Manhattan, New York, without contact with livestock (4). Furthermore, t571 was the only spa type of MSSA identified in a study in the Netherlands of ST398 isolates, including 3 independent cases of nosocomial bacteremia in Rotterdam with no apparent livestock contact (5). spa type t571 was the predominant (11%) MSSA type in patients at a Beijing, China, hospital (6). More recently, a study of t571 MSSA strains from cases of bloodstream infections in France determined that the isolates differed from pig-borne strains and shared similarities with strains from humans in China and virulent USA300 strains (7). These observations concur with a hypothesis that ST398 strains of diverse genotype and geographic origin may also be epidemiologically distinct (8), and livestock contact is a notably inconsistent feature of invasive ST398 infections (5,7–10). The possibility that variants of the ST398 lineage may persist in human populations without livestock contact should not be dismissed. The incidence and severity of clinical infections with ST398 S. aureus in livestock workers as yet have been minimal. Understanding the public health implications of ST398 S. aureus requires systematic investigation of their epidemiology in animals and humans. Human clinical cases of ST398 S. aureus infection should not be indiscriminately attributed to livestock, particularly if isolates are genotypically dissimilar to those occurring commonly in animals.

Staphylococcus aureus nasal carriage, virulence traits, antibiotic resistance mechanisms, and genetic lineages in healthy humans in Spain, with detection of CC398 and CC97 strains

S. aureus nasal carriage was investigated in 278 healthy humans, determining the antibiotic resistance mechanisms, virulence traits, and genetic lineages of recovered isolates. Nasal samples were cultured in specific media for S. aureus and methicillin-resistant S. aureus (MRSA) recovery. S. aureus was detected in 53 of 278 nasal samples (19.1%): MRSA was found in one sample (0.4%) and methicillin-susceptible S. aureus (MSSA) in the remaining 52 samples. The MRSA isolate was typed as ST1649-t701- agrI-SCC mec-IVc and only exhibited resistance to beta-lactams. A high diversity of spa types ( n = 37) was identified among the 52 MSSA, identifying 5 new spa-types. Multi-locus sequence typing (MLST) typing was performed in 30 selected MSSA, detecting 16 different sequence types, 2 of them being new. MSSA strains presented agr types I (30.2%), II (30.2%), III (34%), and IV (5.6%). Eleven strains showed erythromycin resistance and harbored different combinations of erm(A), erm(B), erm(C), erm(T), and msr(A) genes. Two strains exhibited ciprofloxacin resistance, and one of them presented amino acid changes in GyrA and GrlA proteins. The presence of 28 genes encoding staphylococcal toxins was investigated by PCR in all 53 S. aureus isolates. The toxic shock syndrome toxin 1 (TSST-1) gene was detected in 15 MSSA isolates (11 of them typed within the clonal complex CC30) and the gene of exfoliative toxin A in 2 strains. Different combinations of enterotoxin genes were identified among S. aureus strains. None of the S. aureus isolates harbored the Panton-Valentine leukocidin gene. Two MSSA presented the sequence-type ST398 [harboring erm(T) gene], and 2 additional isolates were typed as ST97. Interestingly, MSSA CC398 and CC97 isolates were detected. These clonal complexes are associated with food-producing animals.

Determination of antimicrobial susceptibility patterns and inducible clindamycin resistance in Staphylococcus aureus strains recovered from southeastern Turkey

In this study, we determined the susceptibility patterns of Staphylococcus aureus strains to various antimicrobials and prevalence of inducible clindamycin resistance (ICR) in these isolates. Two hundred and one S aureus strains, isolated from various clinical samples, were included in the study. Antibiotic susceptibilities were studied by disc diffusion method on the basis of the guidelines by the Clinical and Laboratory Standards Institute. The disc diffusion induction test (D test) was applied to determine ICR resistance among erythromycin-resistant S aureus isolates. Of the 201 S aureus strains, 101 (50.2%) were resistant to methicillin. All strains were susceptible to vancomycin, teicoplanin, quinupristin/dalfopristin, and linezolid. It was found that 54 (53.4%) methicillin-resistant S aureus (MRSA) strains were erythromycin resistant, and 40 (39.6%) of them showed constitutive clindamycin resistance. ICR was detected in seven (6.9%) MRSA strains. It was found that 13 (13.0%) methicillin-susceptible S aureus (MSSA) strains were erythromycin resistant. Constitutive clindamycin resistance was seen in one (1.0%) MSSA strain, and ICR was detected in 10 (10.0%) cases. There was a high rate of methicillin resistance among S aureus strains in our hospital. However, no statistically significant difference of ICR was observed between MRSA and MSSA strains (p=0.434) or between inpatients and outpatients (p=0.804). It was concluded that ICR should be routinely evaluated in each S aureus case to avoid therapy failure among patients.

Methicillin-susceptible Staphylococcus aureus from clinical and community sources are genetically diverse

Despite the association of methicillin-susceptible S. aureus (MSSA) with several life-threatening diseases, relatively little is known about their clinical epidemiology in Malaysia. We characterized MSSA isolates (n=252) obtained from clinical and community (carriage) sources based on spa sequencing and multilocus sequence typing (MLST). The prevalence of several important virulence genes was determined to further define the molecular characteristics of MSSA clones circulating in Malaysia. Among the 142 clinical and 110 community-acquired MSSA isolates, 98 different spa types were identified, corresponding to 8 different spa clonal clusters (spa-CCs). In addition, MLST analysis revealed 22 sequence types (STs) with 5 singletons corresponding to 12 MLST-CCs. Interestingly, spa-CC084/085 (MLST-CC15) (p=0.038), spa-non-founder 2 (MLST-ST188) (p=0.002), and spa-CC127 (MLST-CC1) (p=0.049) were identified significantly more often among clinical isolates. spa-CC3204 (MLST-CC121) (p=0.02) and spa-CC015 (MLST-CC45) (p=0.0002) were more common among community isolates. Five dominant MLST-CCs (CC8, CC121, CC1, CC45, and CC5) having clear counterparts among the major MRSA clones were also identified in this study. While the MSSA strains are usually genetically heterogeneous, a relatively high frequency (19/7.5%) of ST188 (t189) strains was found, with 57.8% of these strains carrying the Panton–Valentine leukocidin (PVL). Analysis of additional virulence genes showed a frequency of 36.5% and 36.9% for seg and sei and 0.8% and 6.3% for etb and tst genes, respectively. Arginine catabolic mobile element (ACME) was detected in 4 community isolates only. These represent the first isolates harbouring this gene in an Asian region. In conclusion, MSSA from the Malaysian community and their clinical counterparts are genetically diverse, but certain clones occur more often among clinical isolates than among carriage isolates and vice versa.