Dissemination of resistance plasmids among gentamicin-resistant enterobacteria from hospital patients

Changes in antibiotic resistance of the most common Gram-negative bacteria isolated in intensive care units

We studied the changes in antibiotic resistance of the most common Gram-negative bacteria isolated in the intensive care units at our hospital in 2000 and 2002. Bacterial identification was performed by use of the VITEK 60 analyser, and antibiotic susceptibilities were tested by the VITEK 60 analyser and the disk diffusion agar method. The bacteria isolated most frequently were Pseudomonas aeruginosa (132 strains in 2000 and 106 in 2002), Acinetobacter calcoaceticus (98 and 109 strains, respectively) and Klebsiella pneumoniae (53 and 83 strains, respectively). Acinetobacters presented the highest percentage resistance, with significant increases in resistance to imipenem (15% in 2000 and 67% in 2002) and piperacillin/tazobactam (41% and 72%, respectively). P. aeruginosa presented a significant increase in resistance to all antibiotics, except ceftazidime. A large increase was observed in the resistance of K. pneumoniae to amikacin (from 10% to 50%), ceftazidime (from 80% to 90%) and tobramycin (from 80% to 90%). No imipenem-resistant strains of K. pneumoniae were found.

Hygienic hand disinfection for the removal of epidemic vancomycin-resistant Enterococcus faecium and gentamicin-resistant Enterobacter cloacae

An outbreak strain of Enterococcus faecium, bearing plasmid-mediated vancomycin resistance, and an epidemic gentamicin-resistant, multiply-resistant strain of Enterobacter cloacae, both survived well on fingertips of three volunteers for up to 30 minutes after inoculation. Hand disinfection of inoculated fingers with 60% isopropyl alcohol, or with alcoholic chlorhexidine, reliably gave a 4 log10 reduction of both test organisms. Enterobacter cloacae could not be detected in any finger washings, even by enrichment culture. Chlorhexidine digluconate and povidone-iodine were also effective against E. faecium and Ent. cloacae, giving 4 log10 reductions, but finger washings taken after disinfection yielded low counts of the test strains, e.g. less than 45 recoverable colony forming units (cfu) per finger. Handwashing with soap and water was the least reliable method. The epidemicity, serious antimicrobial multiple resistance and survival on finger-tips of these two strains justifies the use of handwashing agents which have maximum effect. The rapid bactericidal (and residual) activity of alcoholic chlorhexidine suggests that, on the basis of present evidence, it is the preferred agent for hygienic hand disinfection against such strains.

Endogenous colonization by gentamicin-resistant gram-negative bacilli elaborating aminoglycoside (3)-5-acetyltransferase

Members of the family Enterobacteriaceae that elaborate aminoglycoside 3-5-acetyltransferase [AAC(3)-5] caused a nosocomial outbreak at the Vanderbilt University Medical Center and have persisted. To see whether the gene for AAC(3)-5 was present in the community, stool cultures of newly admitted patients and ambulatory persons were examined with a specific gene probe. AAC(3)-5-positive strains were present in the intestinal flora examined.

Prevalence and mechanisms of aminoglycoside resistance. A ten-year study

Aminoglycoside resistance was monitored at St. Thomas' Hospital from 1975 to 1984. Gentamicin resistance had appeared in a number of species by 1975, but remained rare (less than 1 percent of isolates) in Escherichia coli throughout the study period. Gentamicin-resistant Klebsielleae had become fairly common (8 percent of isolates) by 1977, but little change has subsequently occurred in their frequency of isolation. Serratia species are not frequently isolated; gentamicin resistance in these organisms was not observed until 1979. Since then, 10 to 20 percent of isolates have been found to be resistant. Except for Providencia, most isolates of which were gentamicin-resistant, less than 5 percent of the Enterobacteriaceae isolated were found to be resistant to gentamicin during the 10-year period. Throughout the study, approximately 5 percent of the Pseudomonas aeruginosa isolates were resistant to gentamicin. Less than 5 percent of the isolates of Acinetobacter were resistant to gentamicin before 1979, at which time 40 percent were found to be resistant; subsequently, gentamicin resistance among these organisms has become somewhat less common. On the whole, tobramycin resistance has mirrored gentamicin resistance. However, before 1979, most gentamicin-resistant Klebsielleae isolates had retained susceptibility to tobramycin, as had most gentamicin-resistant isolates of Acinetobacter and P. aeruginosa. Amikacin resistance has remained very unusual in all organisms, apart from non-aeruginosa Pseudomonas species. Until 1977, nearly all the resistance among Enterobacteriaceae was attributable to AAC(3)-I, except for that caused by AAC(2') production in Providencia and the non-enzymatic resistance observed in E. coli. However, more recently, AAC(3)-II and AAD(2'') have become the most common mechanisms of resistance. The resistance of most gentamicin-resistant isolates of P. aeruginosa from 1974 to 1977 was attributable to non-enzymatic mechanisms; subsequently, such resistance was more often caused by AAC(3)-I, AAC(6'), or AAD(2''). Gentamicin resistance first appeared in Staphylococcus aureus in 1976, after which about 1 to 2 percent of the isolates from hospitalized patients were found to be resistant, mostly because of production of AAC(6') and APH(2'').

Review of epidemic aminoglycoside resistance worldwide

Epidemic aminoglycoside resistance may be caused by the spread of a species with distinctive chromosomal genes (e.g., Pseudomonas aeruginosa), or it may be due to the dissemination of plasmids or transposons between genera. Although strains of P. aeruginosa resistant to aminoglycosides because of impermeability may cause nosocomial outbreaks, most of the acute increases in aminoglycoside resistance are due to the spread of inactivating enzymes by plasmids. The index species for intergeneric outbreaks is usually Klebsiella pneumoniae carrying the ANT(2") or AAC(3) gene; however, the distribution of resistance varies greatly by location and species. The AAC(6')-I gene is most common in Serratia marcescens and in East Asian isolates of other species, whereas the AAC(3) gene is common in Chile. In the United States, the ANT(2") and AAC(3) genes are particularly common among Enterobacteriaceae, except for Proteus and Providencia, which often carry the AAC(2') gene. The most common patterns of epidemic resistance lead to the inactivation of gentamicin and, less frequently, tobramycin, but only rarely affect amikacin.

Dissemination of gentamicin-resistant plasmids among strains of Enterobacteriaceae

The frequency of gentamicin-resistance among strains of many species of Enterobacteriaceae isolated from in-patients in our hospital has increased considerably since 1979. To clarify the reason for this increase, we tried to detect R-plasmids conferring gentamicin-resistance in all of the isolates in 1981 and found that most of them had conjugative R-plasmids. For epidemiological purposes we classified these into types I to X and 'others' by the following criteria: the nature of the resistances conferred; their incompatibility group; their molecular weight and the pattern of the DNA fragments after digestion with restriction endonucleasis. In the urology department, type-I plasmids conferring resistance to gentamicin, kanamycin, tobramycin and amikacin, and type-II plasmids conferring resistance to gentamicin, kanamycin and tobramycin were detected in the isolates belonging respectively to six species and twelve biotypes, and four species and six biotypes. In the second surgery ward, isolates belonging to six species and twelve biotypes were found to have type-III plasmids conferring resistance to gentamicin, kanamycin, tobramycin and chloramphenicol. Our investigation shows that not only antibiotic-resistant strains but also R-plasmids may be disseminated in hospitals.

Environmental factors contributing to antibiotic resistance

The genetic determinants of antibiotic resistance originate as a result of random events, affected only by those physical and chemical agents that stimulate mutation. It is the selection and spread, within and between species, of new genetic material that is influenced by environmental factors, though the intrinsic antibiotic resistance of many opportunistic pathogens remains to be explained. Depending on the frequency of genetic change, the biochemical resistance mechanism, the efficiency of selection (including access of antibiotic to relevant bacteria), and the opportunity for person-to-person spread, resistance may present in a number of ways. The use of an antibiotic in any individual may give rise to resistance, among pathogens or normal flora, to that antibiotic alone, to some or all members of its class, or to additional unrelated antibacterials, or to the acquisition of new bacteria, from a variety of sources (humans, animals, inanimate environment). A resistant organism may occasionally have other advantages that enable it to spread in the absence of the initial selective agents. In the absence of controlled experiments, which are difficult or perhaps impossible, the only evidence of the truth of these hypotheses comes from the clinical use of antibiotics. A wise observer will conclude that the emergence of resistance is a rare event in an individual patient, but is commoner but by no means universal in populations, especially those that are closed. He will note the reports of diminished resistance in the presence of increased usage. The role of academic infection control is to explain all the phenomena observed, and to formulate methods for identifying and countering the actual as opposed to the imagined hazards.