[New R-plasmids in strains of Serratia marcescens with multiple drug resistance]

In 4 S. marcescens polyresistant strains isolated from patients conjugative plasmids transferred to Escherichia coli have been detected. Two of these strains carry each one plasmid which codes resistance to 10 different antibiotics, including aminoglycosides which rarely occur in our country, and belongs to group IncC. The third strain is the host of 2 plasmids. One of them is similar to the above-mentioned 2 plasmids with respect to the incompatibility group and a set of markers, but additionally codes resistance to cephalosporins; the second plasmid has been determined as belonging to group IncM, unstable and capable of rendering the cells highly resistant only to aminoglycosides. And, finally, the fourth strain also carries 2 plasmids: one of them is unstable and belongs, supposedly, to group IncI alpha, and the second plasmid is stable and belongs to group IncM. The plasmid of group IncI alpha differs from all other plasmids of our Serratia by its capacity of rendering the cells highly resistant to chloramphenicol.

Beta-lactam antibiotics increase the frequency of plasmid transfer in Staphylococcus aureus

The transfer of a plasmid specifying tetracycline resistance between different derivatives of Staphylococcus aureus by phage mediated conjugation was enhanced 100- to 1000-fold by exposure of the culture to subinhibitory concentrations of beta-lactam agents. A variety of other antibiotics, including vancomycin and teicoplanin, had no such effect. The enhanced frequency of transfer was probably due to the formation of large bacterial aggregates.

[Biochemical characteristics of the determinants of drug resistance to gentamycin from clinical strains of gram-negative microorganisms]

Three to four aminoglycoside inactivating enzymes were detected in cell-free extracts of clinical strains of gram-negative bacteria including Pseudomonas, Escherichia, Serratia, Enterobacter and Klebsiella and in cell-free extracts of their transconjugants. The clinical strains were selected by the feature of gentamicin resistance. All the strains contained AAC(3), APH(3') and APH(3"). In addition to these enzymes 12 out of 25 investigated strains contained AAD(2"). The biochemical characteristics of the gentamicin resistance determinant cloned from the clinical strains on the plasmid vector pUC 19 were studied. By the size of the insertion element pAA4 was the smallest among the constructed hybrid plasmids determining gentamicin resistance. It was shown that just this plasmid determined formation of the two gentamicin inactivating enzymes: AAC(3) and AAD(2") in the transformants carrying it.

[Use of DNA-DNA hybridization methods for studying the epidemiology and evolutionary similarity of drug resistance determinants]

A procedure for rapid mass assay of clinical strains of microorganisms based on DNA-DNA hybridization was developed. Epidemiology of the gentamicin resistance determinant cloned earlier from clinical strains of the same hospital was studied and its plasmid localization in representatives of various genera of Enterobacteriaceae was confirmed. The studies on dot-hybridization showed that in three antibiotic-producing organisms, i.e. S. fradiae 918 producing neomycin, M. purpurea 1535 producing gentamicin and S. rimosus 65 producing oxytetracycline there were DNA sequences homologous to those of the gentamicin resistance determinant cloned from the clinical isolates.

[Microbial resistance of Salmonella agona from various regions of Brazil]

The object of the investigation was the evaluation of the susceptibility to antibiotic and chemotherapeutic agents of 240 strains of Salmonella agona isolated from different sources (human, food and environment) obtained from five Brazilian states (Minas Gerais, São Paulo, Rio de Janeiro, Pernambuco and Rio Grande do Sul). The presence of R factors in 26 representative strains of the sample was also determined.

In vitro studies of plasmid-mediated penicillinase from Streptococcus faecalis suggest a staphylococcal origin

A strain of Streptococcus faecalis with plasmid-mediated penicillinase production was studied further. Partially purified penicillinase from the S. faecalis strain hydrolyzed penicillin, ampicillin, and ureido-penicillins but not penicillinase-resistant semisynthetic penicillins, cephalosporins, or imipenem; hydrolysis was inhibited by clavulanic acid. Hydrolysis of a given antibiotic correlated with a marked increase in the minimal inhibitory concentration (MIC) of that drug when a high inoculum was used. As with most enterococci, the MICs of cephalosporins and penicillinase-resistant semisynthetic penicillins were too high for clinical usefulness, although these agents did not show an inoculum effect. Based upon hybridization under stringent conditions of plasmid DNA from the S. faecalis strain to cloned penicillinase genes from Staphylococcus aureus, it appears that these resistance determinants are highly homologous and suggests that this enzyme was introduced into streptococci from staphylococci.

[Transmissible formaldehyde resistance in Serratia marcescens]

It was possible to isolate a strain of Serratia marcescens from fresh clinical bacterial isolates which was 4-6 times more resistant against formaldehyde than other strains. It was shown that the strain harbours two plasmids with molecular sizes of 58- and 90 Mdal. It was demonstrated by conjugation-, transformation- and plasmid-curing experiments that the formaldehyderesistance is plasmid mediated and transferable to E. coli. It was shown by labelling with 14C-formaldehyde that the resistant strains bind much more formaldehyde than the sensible strains.

Increasing resistance to trimethoprim-sulfamethoxazole among isolates of Escherichia coli in developing countries

Resistance of Escherichia coli to trimethoprim (TMP)-sulfamethoxazole remains at 3%-8% at many medical centers within the United States. In this study a 44% resistance rate was observed among E. coli isolated at a pediatric hospital in Santiago, Chile, and a 40% resistance rate at a general teaching hospital in Bangkok, Thailand. Most isolates were from urinary tract infections and showed high-level resistance (minimal inhibitory concentration of TMP greater than 1,000 micrograms/ml). Nineteen of 35 isolates tested transferred resistance to TMP; most cotransferred resistance to streptomycin and sulfonamides. Dihydrofolate reductase type I was detected by gene probing in 14 of 35 strains. Subsequent investigations in Brazil, Honduras, and Costa Rica revealed that this high rate of resistance was not an isolated phenomenon.

[Selection of R plasmid-carrying E. coli pathogens in a mixed population by subinhibitory antibiotic concentrations]

The effect of subinhibitory concentrations of tetracyclin, chloramphenicol, gentamicin and ampicillin in a mixed culture of E. coli was investigated in a chemostat in order to determine minimal selection concentration of these drugs. The mixed culture consisted of three clones of E. coli K12 921. One of these clones was plasmid free, one harboured the F'lac plasmid and the third one an R-factor. The antibiotics tetracyclin, chloramphenicol and gentamicin selected the antibiotic resistant R-factor bearing strain at a concentration of 1/10 of the minimal inhibition concentration. Subinhibitory concentrations of ampicillin failed to select the R-factor bearing strain because this strain produced a secreted betalactamase.

Increasing incidence of resistance to gentamicin and related aminoglycosides in Salmonella typhimurium phage type 204c in England, Wales and Scotland

Phage type 204c of Salmonella typhimurium (DT 204c) appeared in bovine animals in 1979. It is now the predominant type in cattle in England, Wales and Scotland and ranks in the 10 most common phage types in humans. All strains of DT 204c have been resistant to at least four antimicrobial drugs. In 1979 and 1980 the most common resistance pattern was that of chloramphenicol, streptomycin, sulphonamides, tetracyclines and trimethoprim (CSSuTTm) but since 1981 strains with additional resistance to ampicillin and neomycin-kanamycin (AK) have predominated. Strains resistant to furazolidone (Fu) have caused sporadic outbreaks. Gentamicin resistance (G) appeared in DT 204c in 1983 and gentamicin-resistant strains are increasing in incidence. With the exception of resistance to furazolidone, drug resistance in DT 204c has been plasmid-mediated. Characterisation of gentamicin resistance plasmids in DT 204c of R-type ACGKSSuTTm has demonstrated the existence of three distinct lines, two of which have been found exclusively in cattle and one in cattle and humans. The misguided and often inappropriate use of antimicrobial drugs in calves has contributed to the appearance of multiresistant strains of DT 204c and positive measures to limit range and levels of antimicrobials available to feed manufacturers may be necessary.

Isolation and characterization of mutants of the RP4 plasmid coding for increased resistance to ampicillin

E. coli strain J53(RP4) was mutagenized with ethyl methanesulfonate and N-methyl-N'-nitro-N-nitrosoguanidine. Clones showing a two-to threefold increase in resistance to ampicillin were produced. This increase was not due to an increased number of RP4 copies per chromosome. The level of penicillinase activity was twice higher in comparison with the parental strain. No detectable changes were found in the region coding for the resistance to ampicillin on the plasmid by restriction analysis.

[Formation of drug resistance in Vibrio cholerae of non-O1 group inhabitating surface-water reservoirs]

In the study of 255 V. cholerae strains unrelated to vibrio O group I (NAG vibrios), which were isolated from water bodies in the region of the Volga delta in 1977-1982, antibioticograms of 17 types were obtained and R factor was detected in 37.7% of the strains under study. The pronounced heterogeneity of NAG vibrio populations, evaluated in minimal inhibitory concentration (MIC) values with respect to different antibiotics, is specifically manifested by the presence of various dominating subpopulations of these microorganisms, depending on their sensitivity to some chemical drugs. Various representatives of the genus Vibrio were used as a model for demonstrating the ability of the microbial population to enhance its variability as regards MIC values with a seasonal rise in the number of microorganisms.

Inhibition of conjugal transfer of R plasmids by nitrofurans

Nifurzide is a nitrofuran with antibacterial activity. As nitrofurans have been reported to interact with DNA, we tested the ability of nifurzide to inhibit plasmid transfer. Inhibition of plasmid transfer between Escherichia coli strains was obtained for ten plasmids belonging to nine incompatibility groups. The same effect was observed when plasmid RP4 was harboured in six different members of the Enterobacteriaceae. Inhibition depended on the reduction of the -NO2 group of nifurzide and was obtained with four other nitrofuran derivatives.

Trimethoprim resistance in multiple genera of Enterobacteriaceae at a U.S. hospital: spread of the type II dihydrofolate reductase gene by a single plasmid

The percentage of clinical isolates of several species of Enterobacteriaceae, particularly Escherichia coli and Klebsiella pneumoniae, resistant to trimethoprim (TMPR) has increased gradually at the Brigham and Women's Hospital (Boston) in recent years. Thirty-seven of 42 TMPR isolates from six species of gram-negative bacilli conjugally transferred TMP resistance to K12 E. coli. beta-Lactam resistance cotransferred from 21 of the 37 donors, and sulfamethoxazole (SMZ) resistance cotransferred from five of the 37 donors. Plasmids that encoded TMP resistance either alone or with SMZ resistance had a molecular size of approximately 52.5 kilobases, with identical restriction endonuclease-generated "fingerprints." Plasmids encoding beta-lactam-mediated resistance (beta R) were approximately four kilobases larger and had fragment patterns that were identical for all of the TMPR/beta R plasmids tested and had many restriction endonuclease-generated bands in common with TMPR plasmids. Radiolabeled dihydrofolate reductase (DHFR) probes identified the type II DHFR as the determinant of TMP resistance. In contrast with reports from Europe, TMP resistance in multiple species of Enterobacteriaceae was found to be spread in one hospital by a single, stable conjugative plasmid that has a wide host range and encodes the type II DHFR gene.

[Gentamicin: various pharmacotherapeutic aspects in comparison with other aminoglycosides]

Gentamicin may be used in the treatment of infection with gram-negative bacteria including Pseudomonas spp and Proteus spp. Resistance will only appear in suboptimal or too prolonged courses of treatment and usually is due to 'multi-step mutation'. This resistance may be prevented, among others, by combined treatment with gentamicin and an antibiotic of the beta lactam group. When gentamicin is used correctly, it will have few toxic side-effects. Thus, 3 mg/kg of body weight three times daily will usually be indicated to ensure an optimum therapeutic effect. Parenteral administration of gentamicin would only appear to be useful in cases of bacteraemia and/or bacterial infection of the kidney and/or urinary excretory ducts; in the last-named case, the dose given at one time may be reduced by fifty per cent. Local treatment, the most recent method of which consists in administration by I(ntra-)T(racheal) route, apparently offers more prospects.

Effect of bambermycins, in vitro, on plasmid-mediated antimicrobial resistance

The potential of bambermycins (a growth-promoting antimicrobial approved for turkeys, broilers, and swine) to overcome or control plasmid-mediated antimicrobial resistance was determined in a series of in vitro experiments. Four possible modes of action of bambermycins were studied: synergistic effect with 12 other antimicrobials, elimination of resistance (R) plasmids from Escherichia coli, selective killing or inhibition of E coli carrying R plasmids, and inhibition of R plasmid transfer. Bambermycins had no synergistic activity with the other drugs tested and had little effect on eliminating plasmids from host bacteria. Dependent on plasmid type, bambermycins decreased or increased transfer frequency of R plasmids. Bambermycins also selectively inhibited growth of bacteria harboring certain R plasmids.