Prevalence of extrachromosomal drug resistance. Staphylococci in Danish hospitals during the last decade: factors influencing some properties of predominant epidemic strains

Epidemiologic aspects on antibiotic resistance

The increasing usage of antibiotics has selected for resistant bacteria. Spread of such bacteria may follow mathematical models of infectious diseases, taking into consideration the number of infectious individuals, the number of susceptible individuals, and the effective contact rate between individuals from these two groups. According to calculations of the theoretical epidemic curves, the highest incidence of individuals infected with a resistant bacterial strain is present when the prevalence of patients and carriers is approximately 20-80%. Moreover, the rapidity of the spread of an epidemic increases drastically if the total number of individuals in the exposed group increases and also if the contact rate increases. This means that precautions to stop an epidemic spread of a resistant bacterial strain in a given group of individuals should be undertaken early when the prevalence is below 20%. Efficient precautions consist of cohort isolation, decrease of the number of individuals in exposed groups by subdivision into several smaller groups, and decrease of contact rates by hygienic precautions. Examples are given where such precautions have proven efficient.

Comparison of genetic characteristics of MRSA strains present in a Warsaw hospital in 1992 and 1996

Nine isolates of methicillin-resistant Staphylococcus aureus (MRSA) collected in a Warsaw hospital in 1996 were typed by phenotypic (resistograms) and genotypic (PFGE and plasmid restriction analysis-REAP) methods. Twenty-four (MRSA) strains collected in this hospital during a period of the same duration in 1992 and typed earlier using resistograms and PFGE were also typed by REAP. Comparison of typing results obtained for isolates from 1992 and 1996 showed that strains characterised by PFGE patterns of two distinct types described as specific of the two clonally related groups of Polish MRSA in a multicentre study in 1992 are continuously present in the hospital. However, MRSA strains representing PFGE patterns not observed before were also found within the collection from 1996. REAP typing has proved to have a discriminatory power similar to that of PFGE analysis. Nevertheless, due to the lack of plasmids or difficulties in plasmid DNA isolation in 3 out of 33 studied strains, the typability of REAP turned out to be lower than that of PFGE.

Epidemiology of Staphylococcus aureus bacteremia in Denmark from 1957 to 1990

OBJECTIVE: To investigate the changes in epidemiology of Staphylococcus aureus (SA) bacteremia in Denmark over a 30-year period, where the population has remained stable. METHODS: Bacteriologic and clinical data were generated on 17 712 SA strains from virtually all SA bacteremia cases in Denmark from 1957 to 1990 submitted to our laboratory for phage typing. The data were related to information about population, hospital activity and blood-culturing activity during that period. RESULTS: SA bacteremia cases increased from 3 to 20/100 000 inhabitants per year, with the largest increases in incidence rates for the <1-year and >50-year age groups. While blood-culturing activity increased three-fold during the period, the rate of SA bacteremias actually decreased relative to the number of blood cultures taken. The increase in SA bacteremia cases was mainly due to increases in nosocomial infections for all age groups and was related to the increasing admission rates to Danish hospitals. Major shifts in antibiotic resistance patterns and phage types took place during the period, i.e. a marked reduction in multiresistant (including methicillin-resistant) strains, but could not explain the change in the epidemiology of the infections. CONCLUSIONS: The data indicate that increases in SA bacteremia rates correlated significantly with increasing numbers of admissions to hospitals. The main increase in SA bacteremia rates was represented by nosocomial infection, although increasing blood-culturing activity during the period may have contributed.

Increasing Prevalence of Methicillin-Resistant Staphylococcus aureus in the United States

In the period 1975 to 1981, methicillin-resistant Staphylococcus aureus (MRSA) emerged as an important nosocomial pathogen in tertiary care centers in the United States. To determine if the prevalence of this organism has continued to increase, a questionnaire was sent to hospital epidemiologists in 360 acute care hospitals. A total of 256 (71%) of the 360 individuals responded. Overall, 97% (246/256) of responding hospitals reported having patients with MRSA in the period 1987 through 1989. Respondents in 217 hospitals provided estimates of the number of cases seen in 1987, 1988 and 1989. The percentage of respondents reporting one or more patients with MRSA increased from 88% in 1987 to 96.3% in 1989 (p = .0008). The percent of respondents reporting large numbers (≥50) of cases per year increased from 18% in 1987 to 32% in 1989 (p = .0006). Increasing frequency of large outbreaks was observed in community, community-teaching, federal, municipal and university hospitals.

Antimicrobial resistance of Staphylococcus aureus: genetic basis

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The clinical significance of Staphylococcus aureus bacteriuria

To investigate the rate of occurrence, clinical presentation, predisposing factors and frequency of secondary bacteremia 132 patients with significant Staphylococcus aureus bacteriuria were reviewed retrospectively. Staphylococcus aureus accounted for 3.3 per cent of all positive urine cultures. Most patients were elderly men. The most important predisposing factors in the urinary tract were indwelling catheters (63 per cent), obstruction (56 per cent) and instrumentation or surgery (43 per cent). Bacteremia developed secondary to bacteriuria in all 11 patients (8.3 per cent). For that reason Staphylococcus aureus bacteriuria should be regarded as a hazardous condition, especially in patients with predisposing factors in the urinary tract.

Characterization of Staphylococcus aureus isolates from oral surgical outpatients compared to isolates from hospitalized and non-hospitalized individuals

In 160 patients, 114 strains of Staphylococcus aureus were isolated from gingival mucosa, saliva, throat and nose. The patients were divided into 4 groups: one group from an oral surgery clinic for outpatients, one group from a dental clinic, one group from a general surgery clinic for inpatients and one group from a clinic for chronically ill and aged patients. The highest frequency of staphylococcal carriers was found in the outpatient groups (oral surgery and dental clinic), 55% and 45%, respectively. Antibiotic sensitivity testing revealed the majority of the strains to be penicillin resistant, but sensitive to isoxazolyl-penicillins, clindamycin and lincomycin. 50% of the strains produced penicillinase. About 90% of all strains produced lipase, nuclease and a haemolysin most active on rabbit erythrocytes. Human and sheep erythrocytes were lysed by 70% and 46% of the strains, respectively. 77% of the strains were bacteriolytic active. No strain produced lecitinase or elastase. No significant difference was found between the 4 groups in the formation of any of these factors. Phage-type I and III dominated but there was no correlation to enzyme production or patient groups. Thus the 4 patient groups were colonized with strains of Staphylococcus aureus that showed mainly the same pathogenetic factors.

Increasing occurrence of methicillin-resistant Staphylococcus aureus in the United States

Although several outbreaks of methicillin-resistant Staphylococcus aureus (MRSA) infections have been reported in recent years, the geographic distribution and frequency of MRSA infections in American hospitals is unknown. We conducted a questionnaire survey to determine the magnitude of the problem. Data from 261 hospitals were included in the survey. MRSA were reported by 145 hospitals located in 36 states. Large hospitals reported these organisms significantly more often than small hospitals (p less than .001). University hospitals reported MRSA more often than community or community-teaching hospitals (p less than .001 and p less than .005, respectively). The number of hospitals reporting MRSA increased from 24 in 1975 to 112 in 1980 (p less than .001. Our data suggest that MRSA are widely distributed geographically and that the number of hospitals with these organisms has increased dramatically since 1975.

Properties of methicillin-resistant staphylococci now endemic in Australia

Seventy-eight cultures of multiresistant Staphylococcus aureus (MRSA), obtained from Australian hospitals in 1981, were studied to see whether they possessed novel properties responsible for their epidemic spread. In general, these strains resembled MRSA from other countries and were probably derived from them; in particular, they did not survive desiccation better than other staphylococci. The majority of MRSA produced lipases, which might be responsible for their invasiveness, and all produced high amounts of beta-lactamase which hydrolysed each of the isoxazolyl penicillins. This may account for therapeutic failure with cloxacillin of flucloxacillin. Control of this epidemic should rely principally on general measures against the hospital staphylococcus.

Antibiotic policy and spread of Staphylococcus aureus strains in Danish hospitals, 1969-1974

In the period 1969-1974 a decreasing number of Danish hospital departments recorded epidemic occurrences of Staphylococcus aureus strains, and the local spread of strains was less extensive. Multiple-resistant strains of the 83 A complex were succeeded by type 94 strains, resistant to penicillin only. The level of methicillin resistance fell from 19 to 6%. 12% of methicillin resistant strains are now sensitive to streptomycin and/or tetracyclines; they do not represent a few clones, but belong to a wide spectrum of phage types. The changes follow a reduction in the consumption of streptomycin and tetracyclines not of methicillin or other penicillins. As a contrast to the general reduction of combined resistance to streptomycin and tetracyclines, the strains in dermatological departments, where tetracyclines are commonly used, maintain a high degree of resistance to tetracyclines alone.

The identification of staphylococci in clinical and food microbiology laboratories

A comprehensive review of the methods which have been utilized for the identification of staphylococci is presented. Biochemical characteristics which have assisted in the primary isolation of staphylococci, such as pigmentation, hemolytic activity, the egg yolk phenomenon, and deoxyribonuclease and coagulase production, are also analyzed. The potential applicability of advanced techniques to identify staphylococci, such as the detection of enterotoxin production, base ratio analysis, cell wall analysis, phage typing, and serology, is discussed. The following procedures are recommended for routine use: Idnetification of Staphylococcus sp. (clinical laboratories): microscopic observation, catalase activity, coagulase production, lysostaphin sensitivity, and (optional) facultative growth in thioglycolate medium. Identification of Staphylococcus aureus (food laboratories): microscopic observation, catalase activity, coagulase production, thermonuclease production, and (optional) lysostaphin sensitivity.

Staphylococcus aureus strains isolated in Danish hospitals from January 1st, 1966, to December 31st, 1974

During the years 1966-1974, 167,297 strains isolated from 167,297 patients or staff members in Danish hospitals were registered at Staten Seruminstitu. All the strains were phage-typed and examined for production of a 'Tween'-80-splitting enzyme and resistance to mercuric chloride. 158,236 strains were examined for resistance to antibiotics. Since 1968, a steep decrease in the number of strains resistant to three or more antibiotics (multiple-resistant) and in strains of the 83A complex was noticed. In recent years an increase in strains belonging to phage-group I and in those referred to as miscellaneous and non-identified was registered. The increase in the non-typable strains might be explained by the shift of the concentration of the typing phages from 1000 X RTD to 100 X RTD. It is concluded that at least two factors may have contributed to the reduction of the multiple-resistant strains: an altered antibiotic policy, restricting the use of streptomycin and tetracyclines, and an improved hospital hygiene, diminishing the spread of identical strains within the various departments. However, it is emphasized that the consumption of methicillin is still increasing.

Minocycline resistance in Staphylococcus aureus: effect on phage susceptibility

Tetracycline-resistant strains of Staphylococcus aureus are minocycline sensitive, with the exception of strains susceptible to phages of the 83A/84/85 complex and some methicillin-resistant strains of other phage types. Strains of the 83A/84/85 complex yield mutants with increased minocycline resistance. Transduction of minocycline resistance into the susceptible strain RN 450 was obtained with donor strains possessing either markers for both extrachromosomal tetracycline resistance (tet) and chromosomal tetracycline + minocycline resistance (tmn R), or only for chromosomal tmn R resistance. The chromosomal marker was differentiated from the extrachromosomal marker by the lack of detectable extrachromosomal deoxyribonucleic acid after transfer into strain RN 450, transfer into a rec(+) strain, lack of transfer into rec(-) acceptor strain, and cotransduction with chromosomal determinants for guanine biosynthesis. Both chromosomal and extrachromosomal tetracycline resistance can be induced by tetracycline. Induction by tetracycline of chromosomal tetracycline resistance resulted in simultaneous induction of minocycline resistance. The mutation toward increased minocycline resistance (tmn --> tmn R) is a regulatory mutation toward constitutivity or semiconstitutivity. Constitutive resistance is dominant in tmn R/tet diploids. Transfer of the tet marker does not affect the phage susceptibility of the acceptor strain. The tmn R marker, originating from donor strains of the 83A/84/85 complex, renders strain RN 450 resistant to several typing phages, with the exception of phages of the 83A/84/85 complex. This could possibly account for the phage typing patterns of minocycline-resistant staphylococci.

Microbial development of drug resistance: mechanisms and clinical significance

Bacteria have demonstrated a disconcerting ability to develop resistance to antimicrobial agents nearly as quickly as new compounds become available. During the past two decades the molecular bases of several types of resistance have been elucidated. Mechanisms of resistance include the transference of genetic material either through conjugation (involving direct contact between microorganisms), or indirectly through transduction (involving bacteriophages). In addition to this "infectious" drug resistance, genetic mutations which permit the utilization of new metabolic pathways, and the production of enzymes which can inactivate the antimicrobic have been described. One particularly complex problem has been the ability of many Enterobacteriaceae to develop resistance to multiple antimicrobials simultaneously. The possible effect of such an occurrence is illustrated by the recent epidemic of multiply resistant Salmonella typhi in Mexico. Because the typhoid bacilli shared an identical resistance pattern to an epidemic Shigella dysenteriae type 1 the in vivo interspecies transmission of resistance has been postulated. Understanding the various mechanisms of resistance development should allow more rational use of antimicrobial agents.

Genetic basis, epidemiology, and future significance of antibiotic resistance in Staphylococcus aureus: a review

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Characterization of Staphylococcus aureus in a pediatric burn unit

A one-year study on an endemic strain of Staphylococcus aureus phage type 84/85 in a children's burn unit is described. The endemic strain rapidly colonized the burns and nares of acute patients after admission but was not isolated from a patient on admission. Nonendemic strains of S. aureus found on some new patients were mostly non-phage typable and did not prevail in burns. The endemic strain was rarely isolated from the nares and skin of reconstructive patients or from the nares of hospital personnel. The endemic strain did colonize the oral cavity, normal skin, and intestinal tract of some acute patients. Endemic and nonendemic strains of S. aureus from the burned children were compared in their biochemical activities and antibiotic sensitivities to two groups of S. aureus from one other local and one Danish burns unit. The latter groups of strains represented different combinations of staphylococcal phage group III strains. Each of the four groups of strains differed in production of hemolysins, Tween 80 hydrolysis, egg yolk reaction, and proteolysis of casein and gelatin. All of the strains were uniformly sensitive to gentamicin, oxacillin, and cephalothin. Only 4 of 162 strains tested were methicillin resistant. The endemic S. aureus strains of phage type 84/85 were uniformly resistant to eight other antibiotics including lincomycin and clindamycin. The endemic strain was not the known cause of a clinically documented infection in a group of 82 acute patients studied. The possible role of S. aureus strains of phage group III in burn grafting problems is discussed.

Transduction and elimination of resistance determinants in methicillin-resistant Staphylococcus aureus

Elimination and transduction of drug resistance was examined in methicillin-resistant strains of Staphylococcus aureus. Irreversible spontaneous loss and "curing" by aging of cultures and by treatment with ethidium bromide indicated that the determinants for penicillinase production and chloramphenicol resistance, and probably also for neomycin resistance, were located extrachromosomally. On the other hand, the determinants of resistance to erythromycin, streptomycin, tetracycline, and methicillin could not be eliminated by acridines, ethidium bromide, rifampin, sodium dodecyl sulfate, ultraviolet (UV) irradiation, growth at 43.5 C, aging of cultures, or combinations of these treatments. The stimulation of transduction frequency by UV irradiation of phage in the case of the stable markers, but not in the case of the unstable ones, supported further the hypothesis of chromosomal location of the markers of methicillin, erythromycin, tetracycline, and streptomycin resistance and extrachromosomal location of the determinants for penicillinase production and chloramphenicol resistance. Neomycin resistance could not be transduced. Joint elimination and co-transduction of the determinants for penicillinase production and resistance to chloramphenicol and neomycin were not observed, indicating the location of these markers on separate, mutually compatible plasmids. Co-transduction of chromosomal resistance determinants was usually less than 1%, which makes the location of these genes in a circumscribed area of the chromosome improbable.