Aminoglycoside-resistant Isolates Research Articles

Microarray analysis of antimicrobial resistance genes inSalmonella entericafrom preharvest poultry environment

To detect antimicrobial resistance genes in Salmonella isolates from turkey flocks using the microarray technology. A 775 gene probe oligonucleotide microarray was used to detect antimicrobial resistance genes in 34 isolates. All tetracycline-resistant Salmonella harboured tet(A), tet(C) or tet(R), with the exception of one Salmonella serotype Heidelberg isolate. The sul1 gene was detected in 11 of 16 sulfisoxazole-resistant isolates. The aadA, aadA1, aadA2, strA or strB genes were found in aminoglycoside-resistant isolates of Salm. Heidelberg, Salmonella serotype Senftenberg and untypeable Salmonella. The prevalence of mobile genetic elements, such as class I integron and transposon genes, in drug-resistant Salmonella isolates suggested that these elements may contribute to the dissemination of antimicrobial resistance genes in the preharvest poultry environment. Hierarchical clustering analysis demonstrated a close relationship between drug-resistant phenotypes and the corresponding antimicrobial resistance gene profiles. Salmonella serotypes isolated from the poultry environment carry multiple genes that can render them resistant to several antimicrobials used in poultry and humans. Multiple antimicrobial resistance genes in environmental Salmonella isolates could be identified efficiently by microarray analysis. Hierarchical clustering analysis of the data was also found to be a useful tool for analysing emerging patterns of drug resistance.

Aminoglycosides resistance in clinical isolates of Staphylococcus aureus from a University Hospital in Bialystok, Poland.

Staphylococcus aureus obtained from a University Hospital in Poland were characterized in relation to resistance to aminoglycoside antibiotics and the distribution of the genes encoding the most clinically relevant aminoglycoside modifying enzymes (AMEs). Of a total of 118 S. aureus, 45 (38.1%) isolates were found to be resistant to at least one of the tested antibiotics. All aminoglycoside resistant isolates except one 44 (97.8%) were resistant to kanamycin. The majority of strains 37 (82.2%) and 32 (71.1%) expressed resistance to neomycin and tobramycin, respectively. Eleven strains (24.4%) were resistant to gentamicin or amikacin. All S. aureus strains were sensitive to netilmicin. The most prevalent resistance gene was aac(6')-Ie+aph(2') found in 13 (28.9%) strains and 12 (26.7%) isolates carried ant(4')-Ia gene, whilst aph(3')-IIIa gene was detected in only 7 (15.6%) isolates. Additionally, the ant(6)-Ia and str genes were detected in 14 (31.1%) and 2 (4.4%) strains, respectively. Ten (22.2%) strains resistant to amikacin, tobramycin, kanamycin or neomycin did not harbor any of the above-noted genes.

Ciprofloxacin-Resistant Enterobacteria Harboring the aac(6 ′ )-Ib-cr Variant Isolated from Feces of Inpatients in an Intensive Care Unit in Uruguay

The presence of aac(6′)-Ib-cr is associated with decreased susceptibility to aminoglycosides (kanamycin, amikacin, and tobramycin) and to norfloxacin and ciprofloxacin (9). This allelic variant of aac(6′)-Ib was found to be linked to the extended-spectrum β-lactamase (ESBL) gene blaCTX-M-15 in isolates from many countries (4, 6, 7), while association of aac(6′)-Ib with the blaCTX-M-2 ESBL gene has been widely reported in Uruguay and Argentina (3, 11). In this work we looked for the presence of aac(6′)-Ib and the aac(6′)-Ib-cr variant and their putative ESBL coresistance markers in fecal isolates of enterobacteria resistant to ciprofloxacin and/or ceftazidime from inpatients in an intensive care unit (ICU) in Montevideo, Uruguay. From 1 March to 31 October 2006, 106 patients were admitted to this ICU and followed daily until discharge. Rectal swabs obtained at 1, 4, 7, 10, 13, and 16 days after admission were plated on MacConkey agar plus ceftazidime (4 mg/liter) or ciprofloxacin (2 mg/liter). Enterobacterial isolates were identified by classical methods, including only the first isolate of each bacterial species per patient in this study. Antibiotic resistance profiling, screening, and confirmatory testing for ESBL detection were performed by disk diffusion assay, and results were interpreted following the CLSI guidelines (2). A total of 58/106 patients (55.2%) were colonized with ciprofloxacin- and/or ceftazidime-resistant enterobacteria, and 68 isolates were included in this study. Of these, 48 were resistant to gentamicin and 24 to amikacin (Table ​(Table11). TABLE 1. Main characteristics of the 68 studied isolatesa All aminoglycoside-resistant isolates were screened for aac(6′)-Ib by PCR; amplicons were analyzed by restriction with BstF5I, as described by Park et al. (8). PCR products that were not digested by the enzyme [tentatively assigned to aac(6′)-Ib-cr] were confirmed to contain aac(6′)-Ib-cr by double-strand sequencing. Only two Escherichia coli isolates were positive for aac(6′)-Ib-cr detection. Recalling the observed links between blaCTX-M-15 and aac(6′)Ib-cr (4, 6, 7) and between aac(6′)Ib and blaCTX-M-2, the two aac(6′)-Ib-cr-positive isolates were further analyzed by PCR to detect CTX-M-1 and CTX-M-2 group ESBL genes using previously described primers (3, 5). Both isolates were positive only for CTX-M-1 group genes, identified as blaCTX-M-15 after sequencing. Both isolates were obtained at the time of patient admission into the ICU and showed identical pulsed-field gel electrophoresis patterns (10). Both patients were previously hospitalized before ICU admission, suggesting that this strain could be endemic in the hospital, where it could be horizontally transferred. All the other E. coli isolates yielded different pulsotypes (data not shown) compared with these. PCR assays for the detection of class 1 integrons and ISCR1 elements were performed according to the method of Di Conza et al. (3). Both isolates carried a class 1 integron containing the dfr17 and aadA5 gene cassettes, while ISCR1 elements were not detected. So far we have not been able to transfer these resistance genes, either by transformation or by conjugation. This is the first report of aac(6′)-Ib-cr in Uruguay. In accordance with a previous report (6), blaCTX-M-15 and aac(6′)-Ib-cr do not seem to be associated with class 1 integrons. Demonstration of a link to IS26 as previously reported (1) is pending.

Single-Dose Oral Amoxicillin or Linezolid for Prophylaxis of Experimental Endocarditis Due to Vancomycin-Susceptible and Vancomycin-Resistant Enterococcus faecalis

Endocarditis prophylaxis following genitourinary or gastrointestinal procedures targets Enterococcus faecalis. Prophylaxis recommendations advocate oral amoxicillin (2 g in the United States and 3 g in the United Kingdom) in moderate-risk patients and intravenous amoxicillin (2 g) or vancomycin (1 g) plus gentamicin in high-risk patients. While ampicillin-resistant (or amoxicillin-resistant) E. faecalis is still rare, there is a concern that these regimens might fail against vancomycin-resistant and/or aminoglycoside-resistant isolates. The present study tested oral linezolid as an alternative. Rats with catheter-induced aortic vegetations were given prophylaxis simulating human pharmacokinetics of oral amoxicillin (2- to 3-g single dose), oral linezolid (600 mg, single or multiple oral doses every 12 h), or intravenous vancomycin (1-g single dose). Rats were then inoculated with the minimum inoculum infecting 90% of the animals (90% infective dose [ID(90)]) or with 10 times the ID(90) of the vancomycin-susceptible E. faecalis strain JH2-2 or the vancomycin-resistant (VanA phenotype) E. faecalis strain UCN41. Amoxicillin was also tested with two additional vancomycin-susceptible E. faecalis strains, 309 and 1209. Animals were sacrificed 3 days later. All the tested bacteria were susceptible to amoxicillin and gentamicin. Single-dose amoxicillin provided 100% protection against all four isolates at both the ID(90) and 10 times the ID(90). In contrast, linezolid required up to four consecutive doses to provide full protection against the vancomycin-resistant isolate. Vancomycin protected only against the vancomycin-susceptible strain. The high efficacy of single-dose oral amoxicillin suggests that this regimen could be used for prophylaxis in both moderate-risk and high-risk patients without additional aminoglycosides. Linezolid appears to be less reliable, at least against the vancomycin-resistant strain.

Genetic Relatedness among Extended-Spectrum β-Lactamase-ProducingKlebsiella pneumoniaeOutbreak Isolates Associated with Colonization and Invasive Disease in a Neonatal Intensive Care Unit

Pulsed-field gel electrophoresis typing of 60 extended-spectrum beta-lactamase-producing Klebsiella pneumoniae (ESBLKp) isolates obtained in a neonatal intensive care unit during an outbreak indicated the dissemination of two major bacterial genotypes associated with colonization and invasive disease: one composed by aminoglycoside-resistant isolates and the other by aminoglycoside-susceptible isolates. A urease-negative phenotype was observed among aminoglycoside-resistant ESBLKp. Six pairs of isolates from gastrointestinal (GI) colonization and isolates from invasive disease that occurred 3-23 days later were shown to belong to the same genotype, reinforcing a direct association between colonization and subsequent disease. These data indicate that screening for ESBLKp GI colonization in an outbreak setting may be useful to detect neonates at a higher risk of invasive disease.

High-level aminoglycoside resistant enterococci isolated from swine

Approximately 42% (187/444) of swine enterococci collected between the years 1999 and 2000 exhibited high-level resistance to gentamicin (MIC > or =500 microg/ml), kanamycin (MIC > or =500 microg/ml), or streptomycin (MIC > or =1000 microg/ml). Eight aminoglycoside resistance genes were detected using PCR, most frequently ant(6)-Ia and aac(6')-Ii from Enterococcus faecium. Twenty-four per cent (45/187) of total high-level aminoglycoside-resistant isolates and 26% (4/15) of isolates resistant to high levels of all three antimicrobials were negative for all genes tested. These data suggest that enterococci isolated from swine contain diverse and possibly unidentified aminoglycoside resistance genes.

Contribution of the MexXY multidrug transporter to aminoglycoside resistance in Pseudomonas aeruginosa clinical isolates.

MexXY is an aminoglycoside-inducible multidrug transporter shown to contribute to intrinsic and acquired aminoglycoside resistance in laboratory isolates of Pseudomonas aeruginosa. To assess its contribution to aminoglycoside resistance in 14 clinical isolates demonstrating a panaminoglycoside resistance phenotype unlikely to be explained solely by aminoglycoside modification, expression of mexXY by these isolates was examined by reverse transcription-PCR. Elevated levels of mexXY expression were evident for most strains compared with those detected for an aminoglycoside-susceptible control strain, although there was no correlation between mexXY levels and the aminoglycoside MICs for the resistant strains, indicating that if MexXY was playing a role, other factors were also contributing. Deletion of mexXY from 9 of the 14 isolates resulted in enhanced susceptibilities to multiple aminoglycosides, confirming the contribution of this efflux system to the aminoglycoside resistance of these clinical isolates. Still, the impact of MexXY loss varied, with some strains clearly more or less dependent on MexXY for aminoglycoside resistance. Expression of mexXY also varied in these strains, with some showing high-level expression of the efflux genes independent of aminoglycoside exposure (aminoglycoside-independent hyperexpression) and others showing hyperexpression of the efflux genes that was to a greater or lesser degree aminoglycoside dependent. None of these strains carried mutations in mexZ, which encodes a negative regulator of mexXY expression, or in the mexZ-mexXY intergenic region. Thus, mexXY hyperexpression in aminoglycoside-resistant clinical isolates occurs via mutation in one or more as yet unidentified genes.

Mechanisms of antibiotic resistance in Escherichia coli isolates obtained from healthy children in Spain.

Antibiotic resistance and mechanisms involved were studied in Escherichia coli isolates from fecal samples of healthy children. Fifty fecal samples were analyzed, and one colony per sample was recovered and identified by biochemical and molecular tests. Forty-one E. coli isolates were obtained (82%). MIC testing was performed by agar dilution with 18 antibiotics, and the mechanisms of resistance were analyzed. Ampicillin resistance was detected in 24 isolates (58.5%), and blaTEM, blaSHV, and blaOXA type genes were studied by PCR and sequencing. The following beta-lactamases were detected (number of isolates): TEM (20), SHV-1 (1), and OXA-30 (1). The number of aminoglycoside-resistant isolates detected was as follows: streptomycin (15), tobramycin (1), gentamicin (1), and kanamycin (4). The aac(3)-IV gene was detected in the only gentamicin-resistant isolate. Nine (22%) and 2 (5%) isolates showed nalidixic acid (NALR) and ciprofloxacin resistance (CIPR), respectively. Mutations in GyrA and ParC proteins were shown in both NAL(R)-CIP(R) isolates and were the following: (1) GyrA (S83L + D87N), ParC (S801); and (2) GyrA (S83L + A84P), ParC (S80I + A108V). A single mutation in the S83 codon of the gyrA gene was found in the remaining seven NAL(R)-CIP(S) isolates. Tetracycline resistance was identified in 21 isolates (51%) and the following resistance genes were found (number of isolates): tetA (12), tetB (5), and tetD (1). Chloramphenicol resistance was detected in five isolates (12%). These results show that the intestinal tract of healthy children constitutes a reservoir of resistant bacteria and resistance genes.

Antibiotic resistance of enterococci isolated at a teaching hospital in Kuwait

Enterococci isolated in a teaching hospital were studied for their resistance to different antibiotics. Minimum inhibitory concentrations to high-level aminoglycosides and glycopeptide antibiotics were determined by agar dilution and E-test methods respectively. Genes encoding aminoglycoside-modifying enzymes were detected by the polymerase chain reaction (PCR). 195 enterococci were isolated from urines (54.3%), wounds (16.4%), blood (10.2%), and miscellaneous sources (18.9%). They consisted of E. faecalis (88.7%), E. faecium (9.2%), E. casseliflavus (1.5%) and E. bovis (0.5%). None of the enterococci produced penicillinase but 3.5% of them were resistant to ampicillin. They were also resistant to high-level gentamicin (15.9%), kanamycin (22.0%), streptomycin (21.0%), tetracycline (65.1%), erythromycin (62.6%), ciprofloxacin (36.1%), chloramphenicol (26.1%), vancomycin (3.0%) and teicoplanin (2.0%). Most of the high-level aminoglycoside-resistant isolates contained genes coding the bifunctional aminoglycoside modifying enzymes AAC(6′)-APH(2′′), APH(3′) and ANT(6′) but not the ANT(4′) enzyme. The results demonstrated a low prevalence of vancomycin resistance among Enterococci in this hospital.

Clonal origin of aminoglycoside-resistant Citrobacter freundii isolates in a Danish county.

During 1997, attention was drawn to an increased frequency of aminoglycoside-resistant Citrobacterfreundii in a Danish county, when a total of 24 resistant C. freundii isolates was detected. In this study, 15 such isolates were typed by pulsed-field gel electrophoresis, riboprinting and partial sequencing of the gene encoding translation initiation factor 2. Fourteen of the 15 isolates were identical, as evaluated by their antibiograms and by all these typing methods. This epidemic strain harboured the aminoglycoside resistance genes aac(3)-II and ant(3")-I, with the latter located in tandem with a dihydrofolate reductase gene in a class I integron. The source of the strain remains unresolved. Representative isolates were obtained from various specimens from hospitals and general practice throughout the county, with no evidence of patient-to-patient transmission.

Altered expression of oligopeptide-binding protein (OppA) and aminoglycoside resistance in laboratory and clinical Escherichia coli strains.

Oligopeptide-binding protein (OppA) is the periplasmic component of the major oligopeptide transport system of enteric bacteria. Genetic and biochemical evidence suggests that OppA plays a role in the uptake of aminoglycoside antibiotics in Escherichia coli K-12. Forty-six (82%) of 56 aminoglycoside-resistant mutants of E. coli K-12 selected in vitro had reduced or undetectable OppA levels, as compared with their parent strain. Moreover, nine (36%) of 25 aminoglycoside-resistant clinical isolates of E. coli expressed reduced or undetectable levels of OppA. No decrease in OppA expression was observed among aminoglycoside-sensitive E. coli strains from patients. Twenty-three (42%) of 56 aminoglycoside-resistant mutants of E. coli K-12 and six (24%) of 25 clinical isolates also were deficient for expression of ornithine or arginine decarboxylases, or both, and these deficiencies might negatively affect OppA expression by reducing polyamine synthesis. These results support the view that reduced OppA expression is associated with aminoglycoside resistance in E. coli strains.

Aminoglycoside resistance mediated by the bifunctional enzyme 6'-N-aminoglycoside acetyltransferase-2"-O-aminoglycoside phosphotransferase.

The expression of the bifunctional aminoglycoside inactivating enzyme 6'-N-aminoglycoside acetyltransferase-2"-O-aminoglycoside phosphotransferase is the most important mechanism of high-level aminoglycoside resistance in Staphylococcus and Enterococcus. The enzyme is unique because it presents two different aminoglycoside-modifying activities located in different regions of the molecule. The gene aac(6')-aph(2") which encodes the synthesis of the enzyme is present in Tn4100-like transposons which are inserted both in R plasmids and the chromosomes of aminoglycoside-resistant isolates. The genetic structure of aac(6')-aph(2")-containing isolates indicates that their origin is not clonal, but plasmid conjugation together with multiple insertion events are in the basis of the rapid spread of aminoglycoside resistance among Gram-positive bacteria. There is not any prevalent genetic linkage of aac(6')-aph(2") with other antibiotic-resistance determinant. However, most methicillin resistant Staphylococcus strains present also high-level aminoglycoside resistance as the consequence of constant antibiotic pressure. This situation could change in the next future with the reported reemergence of gentamicin-susceptible MRSA isolates. Recent data show that inhibitors of eukaryotic protein kinases inhibit as well the aminoglycoside phosphotransferase activity. This effect indicates a common structure for these two families of proteins and opens the possibility for a meaningful survey of inhibitors of 6'-N-aminoglycoside acetyltransferase-2"-O-aminoglycoside phosphotransferase useful in clinical practice.

Reemergence of gentamicin-susceptible strains of methicillin-resistant Staphylococcus aureus: roles of an infection control program and changes in aminoglycoside use.

The spread of methicillin-resistant Staphylococcus aureus (MRSA) in our hospital in the 1980s correlated with increasing acquisition of resistance to antibiotics including gentamicin, rifampin, and fluoroquinolones. During the period 1993-1995, there was a major change in clinical MRSA isolates: the percentage of aminoglycoside-resistant MRSA isolates decreased from 75% to 52%, while the proportion of heterogeneous MRSA strains susceptible to gentamicin, rifampin, and tetracycline increased gradually from 4.9% to 27.5%. We used five epidemiological markers (i.e., antibiotyping, phage typing, pulsed-field gel electrophoresis, and restriction analysis of PCR amplified coagulase and protein A genes) to characterize recent isolates. With use of these techniques, we confirmed the persistence of the aminoglycoside-resistant MRSA clone and identified a clone of erythromycin-susceptible strains among the gentamicin-susceptible isolates and found that the remaining strains were diverse. These changes were due to the introduction of various MRSA strains from outside the hospital, while implementation of infection control measures in 1991 could have led to reduced transmission of the aminoglycoside-resistant MRSA strain. Changes in antibiotic prescribing patterns that resulted in reduced selective pressure from gentamicin may have contributed to the spread of gentamicin-susceptible MRSA strains.

Increased left-ventricular mass after losartan treatment

species. In the most widely studied family of integrons, the 3' CS is linked to qacE1 (encoding resistance to quaternary ammonium compounds such as chlorhexidine) and sull (encoding sulfamethoxazole resistance). The frequency of occurrence of integrons in bacteria is unknown. In 1994 we had a first outbreak of Klebsiella pneumoniae and K oxytoca infections resistant to gentamicin and cotrimoxazole, isolated from patients with nosocomial infections. Many of these strains were also resistant to other antibiotics. We screened all resistant isolates with a high stringency PCR protocol with oligonucleotide primers specific for the sequence of the published 5' CS and 3' CS regions adjacent to the site-specific recombinational insertion sequence of an integron. This screening enabled amplification of any DNA sequences inserted between the CS regions, if present. Our investigations showed that many bacterial isolates carried integrons with inserted regions of DNA of sizes varying from 800 bp to 3900 bp. Further PCR analysis showed genes encoding for the aminoglycocide-modifying enzymes aadA (encoding resistance to spectinomycin and streptomycin), aadB (encoding resistance to kanamycin, gentamicin, and tobramycin), and aacA7 (encoding resistance to amikacin and tobramycin), inserted as gene cassettes within several of these integrons. Resistance could be transferred experimentally to Escherichia coli by transformation, indicating integrons to be carried on plasmids. Since then, aminoglycoside-resistant Klebsiella isolates have caused sporadic outbreaks in the hospital and resistance to aminoglycosides has subsequently appeared in isolates of Citrobacter freundii and E coli. Many resistant isolates were shown to carry an integron and some had two. The size of the inserted regions of DNA varied from 800 bp to 3900 bp, depending on the number of genes. Sequencing of inserted DNA within three different integrons carried by multiresistant strains of K pneumoniae revealed the presence of a number of resistance genes including aadA, aadB, aacA7, ereA (encoding resistance to erythromycin), dhfrV (encoding resistance to trimethoprim), and catB3 (encoding resistance to chloramphenicol). We have also shown integrons, with various sizes of inserted regions of DNA, in clinical isolates of Enterobacter aerogenes, E cloacae, and Proteus mirabilis (table). Many strains, shown to be unrelated by genotyping, harbour integrons with regions of inserted DNA of identical sizes. This is strongly suggestive of intra-specific and interspecific gene transfer. During the past 3 years, integrons have become widespread throughout our hospital. Their presence has undermined the efficacy of several antibiotics. A review of the published research reveals over 40 known resistance genes to contain the nucleotide sequences associated with

Passage to Nonselective Media Transiently Alters Growth of Mycophenolic Acid-Resistant Mammalian Cells Expressing theEscherichia coliXanthine–Guanine Phosphoribosyltransferase Gene: Implications for Sequential Selection Strategies

TheEscherichia colixanthine–guanine phosphoribosyltransferase gene (Ecogpt) rescues mammalian cells from inhibition of purine nucleotide biosynthesis by mycophenolic acid (MPA). We usedEcogptand other selectable markers to obtain subclones of NIH 3T3 derivatives (EN/NIH) stably expressing transfected genes of interest. In their respective selective mediums, growth of MPA-resistant (MPAR) isolates was indistinguishable from that of aminoglycoside-resistant counterparts expressing selectable marker genes conferring resistance to protein synthesis inhibitors hygromycin B, puromycin, and G418. Growth of aminoglycoside-resistant isolates remained unaltered on passage to nonselective media. In contrast, MPARcells transferred from MPA complete media to nonselective media displayed morphologic changes with static growth. These findings resolved completely by third passage in nonselective media and were independent of the gene of interestcis-linked to the selectable marker. Sequential selection strategies involving cell culture conditions resulting in these altered growth characteristics significantly impaired detection (by selection in G418) of genomic events associated with reactivation of enhancerless, transcriptionally silentneointegrants present in MPAREN/NIH isolates. We explored the cause of these cell culture findings and defined transfection and sequential selection strategies for MPARderivatives that successfully circumvented these effects.

Serotypes and extended spectrum beta-lactam resistance in aminoglycoside resistant Pseudomonas aeruginosa isolates from two Belgian general hospitals: a seven year study

A total of 1896 isolates of Pseudomonas aeruginosa resistant to aminoglycosides and isolated during the period 1983–1989 in two Belgian general hospitals were included in this study. The most frequently encountered O serotypes were O4, O11, O12 and non-typable isolates. The majority of the isolates showed resistance to extended spectrum beta-lactam antibiotics (cefotaxime, ceftriaxone and cefepime). However, a low degree of resistance was found for ceftazidime. By contrast, amikacin and isepamicin, remained active on a significant number of aminoglycoside resistant isolates. In both hospitals, impermeability and AAC(3)II enzyme production were the most prevalent aminoglycoside resistance mechanisms. There were marked differences between the two hospitals with regard to the distribution of the O-serotypes and resistance profiles.

Epidemiology of Pseudomonas aeruginosa in an intensive care unit using selective decontamination of the digestive tract

Selective decontamination of the digestive tract (SDD) aims to reduce the rate of nosocomial infections in critical care patients. Pseudomonas spp. are common nosocomial pathogens and in this study isolates collected from patients and the environment during an SDD trial were examined. The study enrolled 161 SDD cases and 170 controls. Pseudomonads were isolated from 27% of SDD patients and 30% of controls. SDD partially suppressed colonization in the ‘gastro-respiratory’ mucosae but not in the rectum. A total of 108 isolates of pseudomonads were recovered from the environment. Resistance in rectal isolates was minimal but isolates from ‘gastro-respiratory’ sites showed increasing aminoglycoside resistance. Eighty-six per cent of aminoglycoside-resistant isolates from both patient groups and environment were pyocine type 1x. Episodes of infection were reduced in the SDD patients (6) compared with the controls (16), aminoglycoside-resistant strains being associated with zero episodes in SDD patients but with five in the control group.

Speciation, serotyping, antimicrobial sensitivity and plasmid content of Proteeae from the environment of calf-rearing units in South West England.

A survey was undertaken of the occurrence, serotype, antimicrobial sensitivity and plasmid content of members of the tribe Proteeae in the environment of two calf-rearing units in the county of Avon in South West England. Examples of the following species were found: Proteus mirabilis, Prot. vulgaris, Prot. vulgaris Biogroup 2, Morganella morganii, Providencia stuartii, Prov. alcalifaciens and Prov. rettgeri. A wide range of serotypes was found, many having been previously reported from nosocomial isolates. A total of 15% of isolates carried plasmids; six pairs of isolates were identified which had identical serotypes but different patterns of plasmid carriage. The antimicrobial sensitivity of the isolates was generally similar to isolates of Proteeae from humans. Although no truly aminoglycoside-resistant isolates were found, some isolates of Prov. stuartii and Prov. rettgeri had MIC's higher than the other isolates to gentamicin and netilmicin, suggesting the presence of low levels of the enzyme AAC 2'. The study demonstrates that there is a considerable diversity of species and types of Proteeae associated with calves and their environment. It seems likely that a potential cause of colonization of the human gut by Proteeae is the consumption of meat.

Effect of tobramycin on protein synthesis in 2-deoxystreptamine aminoglycoside-resistant clinical isolates of Haemophilus influenzae.

Clinical isolates of Haemophilus influenzae resistant to a broad range of 2-deoxystreptamine aminoglycosides (2-DAM) were studied. The gene responsible for resistance could be mobilized by transformation into a 2-DAM susceptible laboratory strain of H. influenzae, enabling isogenic comparisons. The transformants had the same resistance phenotype as the parental strains. There was close linkage between 2-DAM resistance and streptomycin resistance, a chromosomal marker, but weak linkage between 2-DAM and erythromycin resistance. Resistant transformants exhibited a decreased accumulation of gentamicin due to the absence of the rapid, energy-dependent phase of uptake. Resistance was not through metabolic inactivation of the antibiotic; no aminoglycoside-acetylating, -adenylylating, or -phosphorylating activity was detected in the wild-type strains or in the 2-DAM-resistant transformants. Protein synthesis in 2-DAM-susceptible H. influenzae strains increased in the presence of low (1 microgram/ml) and moderate (50 micrograms/ml) concentrations of tobramycin. With higher concentrations (100 and 500 micrograms/ml), protein synthesis was progressively inhibited. In contrast, protein synthesis in 2-DAM-resistant clinical isolates and transformants was inhibited by 1 microgram of tobramycin per ml, and inhibition increased with higher drug concentrations. Since the stimulating effect of low concentrations of tobramycin in susceptible H. influenzae strains is probably due to misreading, these findings suggest that 2-DAM-resistant strains of H. influenzae have reduced sensitivity to misreading, indicating that altered ribosomes are responsible for the resistance.

Latamoxef in combination with aminoglycosides against Pseudomonas aeruginosa: similarity with ticarcillin.

The in vitro effect of latamoxef against 50 clinical strains of Pseudomonas aeruginosa was compared to that of ticarcillin, both alone and in combination with the aminoglycosides gentamicin, tobramycin and amikacin. Alone, the MIC90 of latamoxef was consistently one-half the MIC90 of ticarcillin. The two antibiotics appeared similar in regard to inoculum effect and bacterial killing. Adding of one-quarter the minimum inhibitory concentration of the aminoglycoside antibiotic to the beta lactam caused reduction in MIC90 of the latter (either ticarcillin or latamoxef) by one-half and decreased the MIC50 by almost one-quarter the concentration required by the beta lactams singly. Therefore, latamoxef singly or in combination with aminoglycosides behaved similarly but was more active than ticarcillin. Using combinations of antibiotics likely to be achieved in the serum of patients, a beneficial in vitro effect (either additive, partially synergistic or synergistic) generally occurred for the beta lactam-aminoglycoside combination if the strain was relatively sensitive to the aminoglycoside used in this combination. It occurred much less frequently for the highly aminoglycoside-resistant isolates.