Oxacillin-susceptible Strains Research Articles

First Report of Oxacillin Susceptible mecA-Positive Staphylococcus aureus in a Children's Hospital in Kunming, China.

PurposeThe present study investigated the prevalence characteristics of oxacillin susceptible mecA-positive Staphylococcus aureus (OS-MRSA) in a children’s hospital in Kunming from January 2019 to December 2020.MethodsA total of 499 S. aureus strains were included in the study and tested for oxacillin susceptibility using the VITEK 2 Compact automated antimicrobial susceptibility test system. All oxacillin-susceptible strains were detected mecA and mecC by polymerase chain reaction (PCR). E-test was used to compare the minimum inhibitory concentration (MIC) values of methicillin-susceptible S. aureus (MSSA), methicillin-resistant S. aureus (MRSA), and OS-MRSA for oxacillin, cefoxitin, penicillin, vancomycin, erythromycin, and clindamycin. Molecular typing of OS-MRSA was performed by MLST and SCCmec typing. Toxin genes were detected by PCR.ResultsForty-five OS-MRSA strains were detected, for an overall rate of 9.02% (45/499). The MICs of MSSA, OS-MRSA, and MRSA against oxacillin were concentrated at 0.38, 0.38, and 12 μg/mL, respectively; the cefoxitin MICs of MSSA and MRSA were concentrated at 2 and 32 μg/mL respectively; and MICs of OS-MRSA were concentrated at 2 and 8 μg/mL; penicillin, vancomycin and erythromycin MICs against MSSA, OS-MRSA, and MRSA showed same centralized points and were 32, 1, and 256 μg/mL, respectively; the MICs of clindamycin against MSSA were 0.5 μg/mL, while that against OS-MRSA and MRSA were concentrated at 256 μg/mL. Molecular typing of OS-MRSA was dominated by ST59-SCCmec IV. The carrier rates of hemolysin genes (hl-a, hl-d) and fibrinogen-binding clumping factor genes (clfA, clfB) were 100% in OS-MRSA, followed by 40% (18/45) for enterotoxin genes (sea, seb).ConclusionOS-MRSA has a high detection rate in children, and main molecular typing is ST59-SCCmecIV in Kunming. The identification ability of automated antibacterial drug sensitivity test detection systems for OS-MRSA is very limited. A combination of phenotypic analysis and molecular detection should be used to improve OS-MRSA identification.

Bacteriological studies on methicillin -resistance Staphylococcus Aureus (MRSA) isolated from cow's mastitis milk

An enormous problem reported worldwide namely bovine mastitis caused by MDR( multi-drug resistant) S.aureus results in prolonged antibiotic treatment and livestock death. Mastitis caused by pathogenic S.aureus strains led to a focused study on surveillance of antibiotic susceptibility along with phenotypic and genotypic characterization. The isolation of Staphylococcus aureus to the determination of antibiotic susceptibility pattern from obtained cow’s milk samples was the aim of the current study. Cows affected by mastitis from different farms in Kalyuobia Governorate, one hundred milk samples were conducted for S.aureus isolation with a prevalence of 30% where 20% isolates were coagulase positive (CoPS) and 10% isolates were coagulase negative CoNS (Gram staining, oxidase, catalase, DNase, haemolysis, and the coagulase test were employed for bacterial identification). The majority of the strains (COPS) n=20 were profiled revealing antimicrobial sensitivity to be multidrug resistant. The occurrence of oxacillin susceptible mecA strains positive strains ( OS-MRSA) was revealed in 6 strains for the first time. Identification of MRSA strain by detection of mecA gene in the PCR .

Short communication: First detection of Panton-Valentine leukocidin–positive methicillin-resistant Staphylococcus aureus ST30 in raw milk taken from dairy cows with mastitis in South Korea

We identified 199 Staphylococcus aureus isolates from quarter milk samples of 1,289 dairy cattle between 2014 and 2018. About 66% of the isolates were resistant to at least 1 antimicrobial agent; the highest rate of resistance was to penicillin, followed by resistance to ampicillin, erythromycin, and sulfadimethoxine. We obtained 30 methicillin-resistant S. aureus (MRSA) strains from 6 farms in 3 provinces. The MRSA strains exhibited a significantly higher resistance rate to most of the tested antimicrobials than the oxacillin-susceptible strains. The MRSA strains represented 5 genotypes: ST72-t324-SCCmec IV (n = 14), ST30-t1752-SCCmec IV (n = 8), ST188-t189-SCCmec NT (n = 6), ST188-t2284-SCCmec NT (n = 1), and NT-NT-SCCmec IV (n = 1). One of the ST188 MRSA strains represented a novel staphylococcal protein A (spa) type (t2284). In addition, 7 of the 8 ST30 MRSA strains were Panton-Valentine leukocidin (PVL)-positive and carried various staphylococcal enterotoxin encoding genes. This is the first report of PVL-positive ST30 MRSA-t1752-SCCmec IV from bovine mastitis in Korea. All of ST72-t324-SCCmec IV MRSA strains carried staphylococcal enterotoxin and leukotoxin encoding genes. They were also sensitive to most of the tested non-β-lactam antimicrobials. In contrast, ST188-t189 MRSA strains were resistant to multiple antimicrobials and predominantly carried the leukotoxin encoding gene. Taken together, these findings may indicate that dairy cows could be a major source for spreading MRSA strains, and contaminated milk could be a vehicle for transmission. Suitable hygienic measures should be established in dairy farms and processing plants to limit the likelihood of introducing MRSA into the food chain.

Koagulaz Negatif Stafilokoklardaki Minimal Inhibitor Konsantrasyon Artisi ile Teikoplanin Kullanimi Arasindaki Iliski

Aim: We aimed to determine the relationship between teicoplanin use and increased minimal inhibitor concentrations of coagulase-negative staphylococci (CNS) isolated from catheter related bloodstream infections (CRBSI) in patients with hematological malignancies. Methods : The study was performed on CNS strains isolated from CRBSI of FN patients during the period between 2006-2010. Teicoplanin MICs were determined by using the Etest method. Demographic characteristics of patients, underlying hematological diseases, the dose and course of teicoplanin were recorded. Grams and international units of teicoplanin were further converted into defined daily doses (DDD). Results: A total of 72 CNS strains causing CRBSI isolated from FN attacks of the patients were analyzed. Among them, Staphylococcus. epidermidis (47%) and Staphylococcus haemolyticus (42%) were the most frequent CNS species. Oxacillin resistance was detected in 74% of all isolates. Increase in MIC values among CNS strains were detected in 44 patients (61%). Mean MIC value for teicoplanin among CNS strains before the treatment was 2.1±1.76 µg/ml, and it was 4.4±3.89 µg/ml after the treatment (p<0.001). Increase in MIC was found to be significantly higher among oxacillin-resistant strains than oxacillin-susceptible strains (p=0,03). A positive correlation was determined between DDD of teicoplanin and MIC increase among CNS strains (p=0.06). Conclusions: We detected an increase in teicoplanin MIC values of CNS isolated from catheter-related BSI related to teicoplanin consumption. The teicoplanin MIC increase was more prominent among oxacillin-resistant isolates. Teicoplanin should be used cautiously in the treatment of repeated CRBSI attacks caused by S. epidermidis and S. haemolyticus which occurred within 90 days after the previous attack.

Nasal colonization of mecA-positive, oxacillin-susceptible, methicillin-resistant Staphylococcus aureus isolates among nursing staff in an Iranian teaching hospital

Methicillin-resistant Staphylococcus aureus (MRSA) is a major cause of health care-associated infections. In this study, a total of 173 nurses was screened for S aureus nasal colonization, of which 8 (4.6%) were MRSA carriers. Among the MRSA isolates, 6 were mecA positive and oxacillin resistant, and 2 were mecA-positive, oxacillin-susceptible (OS-MRSA) strains. Reports of the OS-MRSA strains are increasing worldwide. To the best of our knowledge, this study is the first report on the occurrence of OS-MRSA strains in Iran.

Macrolide–lincosamide–streptogramin B resistance phenotypes in clinical staphylococcal isolates

The prevalence of macrolide–lincosamide–streptogramin B (MLS B) resistance as well as the MLS B resistance phenotypes were investigated by the double-disk diffusion test among 532 clinical staphylococci isolates in a Turkish university hospital. The activity of other antimicrobials, including trimethoprim/sulfamethoxazole, telithromycin, quinupristin/dalfopristin, linezolid, gentamicin, chloramphenicol, ciprofloxacin and vancomycin, was also evaluated. Of 532 isolates, 38.5% were resistant to MLS B antibiotics; 63.9% of the resistant isolates exhibited a constitutive phenotype (cMLS B) whereas 36.1% expressed an inducible resistance phenotype (iMLS B). MLS B resistance was more prevalent among coagulase-negative staphylococci (CoNS) strains. Oxacillin-resistant strains exhibited significantly higher MLS B resistance rates compared with oxacillin-susceptible strains ( P < 0.0001). The most frequently detected resistance phenotype among the total staphylococcal isolates was the constitutive type and this phenotype was more frequently encountered among oxacillin-resistant strains. With the exception of the fully active agents such as vancomycin, linezolid and quinupristin/dalfopristin, the most effective antibiotics were telithromycin and chloramphenicol among all isolates. Susceptibility rates to other antibiotics tested were higher among isolates without MLS B resistance than isolates with MLS B resistance. The detection of a considerable rate (43.5%) of iMLS B resistance among erythromycin-resistant/clindamycin-susceptible strains suggests that the true percentage of clindamycin resistance may be underestimated if testing for inducible resistance is not performed.

Antimicrobial susceptibility of Gram-positive bacterial isolates from the Asia–Pacific region and an in vitro evaluation of the bactericidal activity of daptomycin, vancomycin, and teicoplanin: a SENTRY Program Report (2003–2004)

Medical centres in eight countries in the Asia–Pacific region provided 2391 isolates for the SENTRY Antimicrobial Surveillance Program during 2003–2004 to determine their susceptibility to several antimicrobial classes, including daptomycin. Daptomycin, vancomycin and teicoplanin minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) were determined for 120 isolates of Staphylococcus aureus, which included wild-type (WT) methicillin-resistant S. aureus (MRSA) and strains with decreased susceptibility to vancomycin (hetero-vancomycin-intermediate S. aureus (hVISA)). Oxacillin-resistant staphylococcal isolates were much less susceptible to the other tested agents compared with oxacillin-susceptible strains. Vancomycin resistance was higher among Enterococcus faecium (10.3%) than Enterococcus faecalis (0.4%), and macrolide resistance was high both for β-haemolytic (17.7%) and viridans group (48.7%) streptococci. Daptomycin (MIC for 90% of the organisms (MIC 90) = 0.5–1 mg/L) was two-fold more potent than vancomycin, with >99% susceptibility when tested against staphylococci. All tested isolates of E. faecalis (MIC 90 = 2 mg/L) and β-haemolytic streptococci (MIC 90 = 0.5 mg/L) were susceptible to daptomycin. Daptomycin MIC and MBC values were slightly higher for the hVISA isolates compared with WT-MRSA, with MBC/MIC ratios of only 1–2 for both groups. The MBC/MIC ratio for vancomycin was often greater when tested against these strains, particularly hVISA. In contrast, teicoplanin MBC/MIC ratios were significantly higher, with many of the strains showing values consistent with tolerance (≥32). Daptomycin was demonstrated to have excellent in vitro activity when tested against Gram-positive isolates collected from Asia–Pacific countries, including hVISA strains.

Activity of Dalbavancin Tested against Staphylococcus spp. and β-Hemolytic Streptococcus spp. Isolated from 52 Geographically Diverse Medical Centers in the United States

Dalbavancin is a lipoglycopeptide antimicrobial agent with a potency significantly better than that of vancomycin when tested against staphylococci and streptococci. These two pathogens are common causes of skin and skin structure infections (SSSIs), and dalbavancin has been approved for the treatment of moderate to severe SSSIs. This study generated susceptibility data for staphylococci and beta-hemolytic streptococci from 52 U.S. medical centers that locally tested dalbavancin, vancomycin, and other antimicrobial class representatives to assess the potency of dalbavancin and the overall contemporary activities of commonly prescribed agents. Locally generated data showed that oxacillin-resistant staphylococci (57.0% overall) had slightly higher dalbavancin MIC(90) values (0.19 microg/ml) than oxacillin-susceptible strains (0.125 microg/ml). This potency was 8- to 16-fold greater than that for vancomycin. beta-Hemolytic streptococci had MIC(90) values ranging between <or=0.016 and 0.064 microg/ml (highest for group B isolates). Levofloxacin, gentamicin, and tetracycline were active against oxacillin-susceptible staphylococci (82 to 99% susceptible), with lower susceptibility rates seen for the oxacillin-resistant strains. Linezolid coverage was >98% against staphylococci. Erythromycin resistance was high for staphylococci (30.6 to 94.1%) with inducible clindamycin resistance rates of 26.0% and 55.0% for oxacillin-susceptible and -resistant Staphylococcus aureus, respectively. Beta-hemolytic streptococci had a 20.2% erythromycin resistance rate and a 60% inducible clindamycin resistance rate but were highly susceptible to other tested agents. Etest reading errors were apparent and skewed results towards slightly higher dalbavancin MICs, requiring further education on how to interpret Etest method results for this compound. Current disk diffusion breakpoint criteria for oxacillin susceptibility for S. aureus showed a very-major-error rate of 2.3% and only a 0.9% minor-error rate when cefoxitin was used to predict oxacillin susceptibility. Dalbavancin demonstrated excellent potency and activity (100% susceptibility at proposed breakpoints) against common causes of SSSI pathogens in this U.S. multicenter study sample.

Evaluation of a new chromogenic medium for isolation of MRSA

Methicillin-resistant Staphylococcus aureus (MRSA) is responsible for an increasing number of serious nosocomial and community-acquired infections, and accurate methods to detect such strains are needed. We tested the susceptibility of 3 kinds of MRSA isolation medium, MRSA Screen Agar, Oxacillin Resistance Screening Agar and CHROMagar MRSA. Both sensitivity and specificity of CHROMagar MRSA were 100%. The sensitivity and specificity of both MRSA Screen Agar and Oxacillin Resistance Screening Agar were 100% and 91.5%, respectively. It is suggested that CHROMagar MRSA is a useful medium to detect MRSA including mecA positive and oxacillin susceptible strains.

Species identification, slime production and oxacillin susceptibility in coagulase-negative staphylococci isolated from nosocomial specimens

Ninety-two coagulase negative staphylococci (CNS) (forty-five of clinical origin and forty-seven of environmental origin), collected in a hospital in San Luis, Argentina, from March to June, 1999, were identified to species level by the ID 32 Staph and API Staph System (bioMérieux). Slime production was investigated by the quantitative and qualitative methods. Oxacillin susceptibility was determined by the disk diffusion test (1 µg), the agar dilution method (0.125 to 4 mg/ml) and agar screen (6 µg/ml). The presence of mecA gene was investigated by PCR. The clinical CNS species most commonly isolated were S. epidermidis, S. haemolyticus, S. hominis and S. saprophyticus. The frequency of slime production by clinical and environmental isolates was similar (25/45 and 27/47, respectively) and the results obtained by the quantitative and the qualitative methods correlated well. The mecA gene was detected in all S. epidermidis, S. haemolyticus and S. hominis isolates, which were resistant to oxacillin by the phenotypic methods. However, this gene was not present in S. klossii, S. equorum, S. xylosus and S. capitis strains. The gene was neither found in two out of the six S. saprophyticus isolates, in two out of three S. cohnii subsp. urealyticum isolates and in two out of five S. cohnii subsp. cohnii isolates, all of which resulted oxacillin resistant according to MIC. The gene was not found in oxacillin-susceptible strains either. Most of the CNS isolates (enviromental and clinical) that were slime producers were found to be oxacillin resistant, which makes the early detection of these microorganisms necessary to prevent their dissemination in hospitals, particularly among immunocompromised patients.

Evaluation of a latex agglutination assay for rapid detection of oxacillin resistant Staphylococcus aureus

Oxacillin resistance in Staphylococcus aureus is mediated by the mecA gene, resulting in production of penicillin-binding protein 2a (PBP2a), which is not present in the oxacillin susceptible strains. We evaluated the ability of a 30-min latex agglutination (LA) test (Seiken, Tokyo, Japan) to detect production of PBP2a in 315 clinical isolates of S. aureus. The LA results were compared with results of susceptibility testing using the Vitek GPS-SV test card. The latex test was positive for all 206 isolates determined to be methicillin resistant by Vitek (sensitivity 100%), the latex test was negative for 108 of 109 isolates determined to be oxacillin susceptible by Vitek, and the latex test was positive for 1 isolate determined to be susceptible by Vitek (specificity 99.1%). The discrepant isolate was negative for the mecA gene by polymerase chain reaction (PCR). The LA test is a rapid and reliable method for detecting oxacillin resistant S. aureus.

Staphylococcus aureus and Coagulase-Negative Staphylococci from Blood Stream Infections: Frequency of Occurrence, Antimicrobial Susceptibility, and Molecular ( mecA) Characterization of Oxacillin Resistance in the SCOPE Program

Staphylococci are major causes of nosocomial blood stream infection. The recently completed SCOPE Surveillance Program found that coagulase-negative staphylococci (CoNS) and Staphylococcus aureus were the first and second most common etiologic agents, respectively, causing nosocomial blood stream infection in the USA. The frequency of oxacillin resistance was 68% among 1553 strains of CoNS and 26% among 787 strains of S. aureus in this study. Extended susceptibility profiles were generated for a subset of 150 S. aureus and 300 CoNS against 16 antimicrobial agents. Oxacillin-susceptible strains of both CoNS and S. aureus were uniformly susceptible to β-lactam agents with the exception of ampicillin and penicillin. Oxacillin-susceptible S. aureus were also highly susceptible to the fluoroquinolones, aminoglycosides, and trimethoprim/sulfamethoxazole. The oxacillin-susceptible CoNS were less susceptible to these agents, and only glycopeptides were reliably active against oxacillin-resistant strains. PCR detection of the mecA gene was used to scrutinize current NCCLS interpretive breakpoint MICs for determining susceptibility or resistance to oxacillin. We found complete concordance between the presence or absence of mecA and the NCCLS oxacillin interpretive breakpoint categories for S. aureus. In contrast, the NCCLS breakpoints for oxacillin significantly underestimate the degree of true oxacillin resistance among CoNS. Using the presence of mecA as the reference standard, we detected 15.7% false susceptibility to oxacillin using a MIC susceptible breakpoint concentration of ≤2 μg/mL. Lowering the oxacillin MIC breakpoint to ≤0.25 μg/mL for CoNS would greatly improve the accuracy of the MIC test performance. We found that both the current oxacillin disk test and the 30-μg ceftizoxime disk test functioned quite well in predicting those strains of CoNS that contain mecA. These studies have demonstrated both a high level of antimicrobial resistance among nosocomial blood stream isolates of staphylococci as well as significant problems with the current NCCLS breakpoints for oxacillin when testing CoNS.

Rapid detection of methicillin-resistant staphylococci using polymerase chain reaction

Objective: A comparison was made of polymerase chain reaction (PCR), Southern blot, and routine susceptibility testing for determination of methicillin resistance in clinical isolates of staphylococci. Methods: A PCR-based test and Southern hybridization were used to detect the mecA gene in staphylococci. A broth double-dilution method was used to determine the minimum inhibitory concentration (MIC) for oxacillin. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and then fluorography were used to test the affinities of penicillin-binding proteins of Staphylococcus aureus ATCC 25923 for oxacillin. Results: The presence or absence of a meth icillin-resistant gene ( mecA) in 228 clinical isolates of staphylococci was examined by PCR and Southern blot analyses. The results were compared in relation to those of the MIC assay to oxacillin. A total of 57 of 58 oxacillin-resistant S. aureus strains were mecA-positive, whereas 3 of 126 oxacillin-susceptible strains were mecA-positive. For 21 oxacillin-resistant coagulase-negative staphylococci, 100% of the strains were mecA-positive. but 9 of 23 oxacillin-susceptible coagulase-negative staphylococci were mecA-positive. The PCR test identified methicillin-resistant staphylococci in less than 3 hours, using as few as 300 cells or 3 pg crude extract DNA as the PCR template. The results of the PCR test correlated well with those of DNA hybridization. Dot blot hybridization could detect as little as 4 ng DNA. Five penicillin-binding proteins were identified in S. aureus. Conclusions: Identification of methicillin-resistant staphylococci by PCR (confirmed by DNA hybridization) offers a specific, sensitive, and rapid alternative to traditional susceptibility testing and serves as a guide for the rational treatment of infections caused by staphylococci.

The utility of non-β-lactam antimicrobial MICs as markers to distinguish oxacillin-resistant from oxacillin-susceptible strains of Staphylococcus epidermidis

Among 6,068 strains of Staphylococcus epidermidis, 75.5% were oxacillin-resistant. Oxacillin-susceptible strains were more frequently susceptible to erythromycin, clindamycin, ciprofloxacin, trimethoprim/sulfamethoxazole, gentamicin, and tetracycline than oxacillin-resistant strains. With the exception of erythromycin, non-β-lactam MICs were less discriminatory for identifying oxacillin-resistant strains with oxacillin MICs ⩽2 μg/ml than for those with oxacillin MICs ⩾4 μg/ml.

Emergence of bacterial resistance to imipenem and ciprofloxacin in a university hospital.

We have continuously monitored the in-vitro activities of imipenem and ciprofloxacin against large numbers of non-fastidious clinical isolates. After eight years of use, 97-100% of Enterobacteriaceae and Acinetobacter baumannii remained susceptible to imipenem, but susceptibility of Pseudomonas aeruginosa declined from 100% to 91%. After six years of use, 94%-100% of Enterobacteriaceae (except Providencia stuartii) remained susceptible to ciprofloxacin but susceptibility of P. stuartii, A. baumannii and P. aeruginosa declined from 100% to 46%, 66% and 84%, respectively. Oxacillin-resistant staphylococci were considered to be resistant to imipenem and all beta-lactams. There were no quinolone-resistant staphylococci observed in 1986, but susceptibilities of Staphylococcus aureus, Staphylococcus epidermidis and Staphylococcus haemolyticus to ciprofloxacin decreased to 85-93% for oxacillin-susceptible strains and to 7-39% for oxacillin-resistant strains. Enterococcus faecalis has remained susceptible to imipenem and the modal MIC of ciprofloxacin has remained 1 mg/L; however, susceptibility to ciprofloxacin 2 mg/L decreased from 94% to 64%. Imipenem-quinolone cross-resistance was observed for staphylococci but not for P. aeruginosa.

Cefquinome (HR 111V) in vitro evaluation of a broad-spectrum cephalosporin indicated for infections in animals

Cefquinome (formerly HR 111V), an aminothiazolyl cephalosporin, was compared with cefepime, cefpirome, cefotaxime, and ceftazidime against 681 clinical cultures and a challenge set of bacteria with well-characterized resistance mechanisms. Cefquinome minimum inhibitory concentrations (MIC90) for the enterobacteriaceae ranged from < or = 0.12-2 micrograms/ml with the highest MIC (4 micrograms/ml) obtained among Citrobacter freundii, Enterobacter cloacae, and Providencia stuartii strains. A total of 90% of the Pseudomonas aeruginosa were inhibited by cefquinome at < or = 8 micrograms/ml. Cefquinome activity of particular note for Gram-positive isolates included Corynebacterium jeikeium (MIC90, 8 micrograms/ml) and enterococci (MIC50, 4-8 micrograms/ml). Oxacillin-resistant Staphylococcus aureus was 32-fold less susceptible (MIC90, 16 micrograms/ml) to cefquinome than oxacillin-susceptible (MIC90, 0.5 micrograms/ml) strains. Cefquinome was very potent against fastidious isolates such as Moraxella catarrhalis (MIC90, 0.25-2 micrograms/ml); Haemophilus influenzae (MIC90, 0.06-1 micrograms/ml), Neisseria gonorrhoeae (MIC90, 0.06-0.5 micrograms/ml), and Streptococcus species (MIC90, < or = 0.03-006 micrograms/ml). When tested against organisms possessing Bush group 2 enzymes (including extended spectrum beta-lactamases), cefquinome remained active (MIC, < or = 8 micrograms/ml) against the majority of strains. This compound should be very active against pathogens generally found in animal infections and possesses a potency and spectrum comparable to the "fourth-generation" cephalosporins (cefepime and cefpirome) being investigated for human infectious diseases.

Aorto-femoral graft infections: a clinical and microbiological analysis

A 5-year experience of 19 aortic graft infections is reviewed. Of these, 13 (68%) had a late onset (more than 4 months after graft implantation) and usually presented with anastomotic pseudo-aneurysm or thrombosis. The remaining six infections (32%) had an early onset and presented more often with surgical wound infection. Aorto-enteric fistulae and inguinal sinus tracts were observed in both early and late onset infections. Coagulase-negative staphylococci (all slime negative, oxacillin susceptible strains) were the prevalent pathogens in both groups of infections and were isolated in six (32%) patients. Next most commonly seen were Pseudomonas aeruginosa in four (21%) patients, Enterococcus spp. in three (16%) patients, Staphylococcus aureus in three (16%) patients, other bacteria in six (32%) patients. No organisms were isolated in three (16%) patients. Mortality and major amputation rates were 47.3% and 31.6%, respectively. The therapeutic procedures included total graft removal (15 patients), partial graft excision (two patients), partial graft excision followed by total graft removal (one patient) and local treatment without graft removal (one patient). Six patients recovered, including two who underwent total graft removal associated with a non-conventional 'in situ' graft replacement and one patient treated conservatively with local treatment and antibiotics. The three patients undergoing partial graft excision showed signs of active infection of the residual graft.

Activity of ampicillin-sulbactam and oxacillin in experimental endocarditis caused by beta-lactamase-hyperproducing Staphylococcus aureus

Using a rat model of aortic valve infective endocarditis, we previously found that oxacillin was equally effective against an oxacillin-susceptible strain of Staphylococcus aureus and a beta-lactamase-hyperproducing borderline oxacillin-susceptible strain of S. aureus; also, ampicillin-sulbactam was less effective than oxacillin against both isolates and at low doses was less effective against the borderline-susceptible strain than against the fully oxacillin-susceptible strain (C. Thauvin-Eliopoulos, L. B. Rice, G. M. Eliopoulos, and R. C. Moellering, Jr., Antimicrob. Agents Chemother. 34:728-732, 1990). In the present study, we extended this work, using alternative treatment schedules and additional bacterial strains. Extending treatment with low doses of ampicillin-sulbactam (500 and 250 mg/kg of body weight per day, respectively) to 6.5 days resulted in equalization of effectiveness against the previously studied strains BOSSA-1 and OSSA-1 (3.75 +/- 1.61 log10 and 4.71 +/- 1.79 log10 CFU of residual viable bacteria per g, respectively). Against the borderline oxacillin-susceptible strain BOSSA-1, increasing the sulbactam dosage from 500 to 2,000 mg/kg/day while maintaining a fixed dose of ampicillin (1,000 mg/kg/day) by continuous infusion resulted in lower bacterial counts (4.93 +/- 1.84 log10 versus 3.65 +/- 1.26 log10 CFU of residual viable bacteria per g, respectively), but this difference was of only borderline significance; differences in efficacy between the low-dose and high-dose sulbactam regimens were exaggerated when intermittent intravenous administration was used (6.19 +/- 1.90 log10 versus 3.37 +/- 1.41 log10 CFU/g, respectively; P < 0.001). However, for any individual sulbactam dosage, the model of administration (continuous versus intermittent infusion) did not affect the activity of the regimen. When additional strains were used in the model, oxacillin and ampicillin-sulbactam (1,000 plus 2,000 mg/kg/day) were equally effective against both oxacillin-susceptible and borderline oxacillin-resistant strains of S. aureus. These results support the predictions that oxacillin would be clinically effective in the treatment of infections caused by borderline oxacillin-susceptible strains of S. aureus and that, except at very low doses, ampicillin-sulbactam would also be as effective against borderline-susceptible strains as against fully oxacillin-susceptible strains of S. aureus.

Use of chemiluminescent DNA probes in the rapid detection of oxacillin resistance in clinically isolated strains of Staphylococcus aureus

Chemiluminescent DNA probes (AccuProbe, species specific; and FlashTrack, bacterial generic) were used to determine oxacillin resistance in Staphylococcus aureus. Ribosomal RNA was measured at designated intervals in the presence and absence of antibiotic. A total of 48 (AccuProbe assay) and 24 (FlashTrack) S. aureus isolates with known oxacillin susceptibility patterns were inoculated into Bactec 6A bottles both with and without 4 μg/ml oxacillin and incubated at 35°C for 4 h. Aliquots were removed at 0 and 4 h, and pellets of bacteria were obtained via selective centrifugation. Probe assay counts (relative light units, RLUs) were performed. Of 21 oxacillin-resistant S. aureus (ORSA) strains, 20 showed a >5-fold RLU increase during the incubation period (AccuProbe assay): 25 of 27 oxacillin-susceptible strains demonstrated a ⩽4-fold increase. AccuProbe test sensitivity and specificity were 95% and 92%, respectively. With the generic FlashTrack probe assay, all nine ORSA isolates showed a ⩾4-to 10-fold increase in RLUs, and all 15 oxacillin-susceptible strains showed a ⩽4-fold increase in RLUs during the 4-h incubation. The FlashTrack test sensitivity and specificity were both 100%. Probe assays were completed within 5 h. This study suggests that rapid and reliable determination of oxacillin resistance in S. aureus clinical isolates can be accomplished using commercially available DNA probes.

Beta-Lactam-beta-lactamase-inhibitor combinations are active in experimental endocarditis caused by beta-lactamase-producing oxacillin-resistant staphylococci

Optimal therapeutic strategies for serious infections caused by borderline and heterotypic oxacillin-resistant Staphylococcus aureus (BORSA and ORSA) strains have not been fully characterized. Recent evidence suggests that the dominant penicillin-binding protein of ORSA strains (PBP 2a) shows good affinity for ampicillin and that these strains commonly produce beta-lactamase. Therefore, we compared the in vivo efficacy of the combination of ampicillin plus sulbactam with that of vancomycin against ORSA strains. Also, the moderate resistance of BORSA strains appears to be attributable mainly to the hyperproduction of beta-lactamase. Therefore, we also studied the in vivo efficacy of ampicillin plus sulbactam against such organisms. Experimental aortic endocarditis was induced in rabbits by the following three strains: beta-lactamase-producing BORSA strain VP-986, beta-lactamase-producing ORSA strain 67-0, and its beta-lactamase-negative clone. In animals with BORSA endocarditis, ampicillin plus sulbactam and oxacillin were highly effective in reducing mean intravegetation bacterial densities, with each being significantly better than either ampicillin alone or no therapy. In animals with endocarditis caused by the beta-lactamase-producing ORSA strain, ampicillin plus sulbactam was significantly better at reducing mean vegetation bacterial densities than the other regimens. For endocarditis caused by the beta-lactamase-negative ORSA clone, ampicillin was better than vancomycin in reducing mean intravegetation bacterial densities. These data show that infections caused by beta-lactamase-producing BORSA strains respond therapeutically in a manner similar to that of infections caused by oxacillin-susceptible strains, with both oxacillin and ampicillin plus sulbactam being highly efficacious. Moreover, high-dose ampicillin treatment strategies were effective in the therapy of ORSA endocarditis; this efficacy is presumably related to the relatively high affinity profile of this compound (compare with that of oxacillin) for the functionally dominant ORSA PBP 2a.