Correlation between "methicillin resistance" and serotype in Staphylococcus

A Shared noncapsular antigen is responsible for false-positive reactions by Staphylococcus epidermidis in commercial agglutination tests for Staphylococcus aureus

Many of the commercial slide agglutination tests for Staphylococcus aureus incorporate antibodies against cell surface antigens associated with methicillin resistance, including capsular polysaccharides and an uncharacterized antigen, serotype 18. These tests are more sensitive than the first-generation agglutination procedures that detected only bound coagulase and protein A, but they suffer from false-positive reactions with some coagulase-negative staphylococci. The aim of this study was to elucidate the mechanism for false-positive agglutination by S. epidermidis in these tests. A group of methicillin-resistant S. aureus (MRSA) isolates, including a serotype 18 strain, that were not detectable in the first-generation tests were found to be of capsular polysaccharide type 8. All of these isolates were deficient in bound coagulase and/or protein A, and they possessed a heat-stable, proteinaceous antigen that was absent from a prototype capsule type 8 strain. Enzyme-linked immunosorbent assay and agarose gel immunodiffusion experiments demonstrated that this proteinaceous antigen was also present on both methicillin-sensitive and methicillin-resistant S. epidermidis clinical isolates. S. epidermidis strains that gave false-positive agglutination test results had a considerably higher level of this antigen than strains that gave the correct negative result. These findings reveal the importance of the careful selection of MRSA strains for raising anti-capsular type 8 antibodies for use in agglutination tests. Strains devoid of the antigen shared with S. epidermidis should be used to eliminate potential cross-reactions with this coagulase-negative coccus.

Methicillin-resistant Staphylococcus aureus: implications for the 1990s and effective control measures

Data from around the world verify the escalating incidence of infections caused by methicillin-resistant Staphylococcus aureus (MRSA). Since MRSA are spread primarily on the hands of health care workers, rates of infection are a function of infection control activities within institutions. Moreover, infections with MRSA are serious and often life-threatening. Thus, there are compelling medical and ethical reasons to invest in control measures. Currently available data suggest the efficacy of three measures: (a) identification of the entire patient reservoir (cases and carriers) for purposes of isolation; (b) strict handwashing between patients to prevent transmission; and (c) treatment of the carrier state in health care workers and patients during periods of high infection rates with safe and effective topical agents such as mupirocin.

Early stages of in vitro killing curve of LY146032 and vancomycin for Staphylococcus aureus

The early stages of the time-killing curves of vancomycin and LY146032 have been studied, by use of short sampling intervals, for three strains of Staphylococcus aureus. Both vancomycin and LY146032 killed S. aureus, but the time-killing curves differed: the effect of vancomycin was slow, limited, and not related to the concentration of the drug, whereas that of LY146032 was rapid, extensive, and related to concentration. When strains ATCC 25923 and CIP 6525 were exposed to LY146032, the population decreased exponentially with time. The killing rate was constant and linked to the concentration by a Michaelis-Menten relationship. The maximum killing rate and the affinity constant of LY146032, estimated from the data transformed by the Lineweaver-Burk method, differed for the two strains. The concentration of the antibiotic at which killing theoretically begins (estimated by linear regression using the logarithm of the concentration) is of the same magnitude as the MIC of LY146032, which indicates the pure bactericidal mode of action of the drug. S. aureus ATCC 12600 was more resistant to the bactericidal effect of the two drugs, and its killing curve did not conform to the model described here.

Control at hospital level of infections by methicillin-resistant staphylococci in children

Rapid spread of methicillin-resistant staphylococci (MeRS) in a children's hospital is described. Within 4 months of the first isolation MeRS had been isolated from infections in all clinical units. MeRS were also regularly isolated at the out-patient department. Protective isolation of one of the clinical units had no effect on the infection rate by MeRS. The use of antiseptics (Hexachlorophene and chlorhexidine) and gentamicin nose cream in children and staff members in three out of five clinical units resulted in a significant reduction of the prevalence of nose colonization by MeRS in children. In staff members a non-significant reduction of the prevalence of colonization and a significant reduction of the acquisition of MeRS was found. After a few months the infection rate decreased to zero in the units where the measures were introduced. It remained unchanged in the other units. Phage typing of two sets of strains collected at an interval of 6 months showed that the infections were mainly caused by two endemic strains of MeRS. The majority of the infections caused by MeRS was of minor importance. In 16% of the infections a strain was isolated repeatedly and for more than 1 week. After the introduction of antiseptics a relative increase of infections by Gram-negative bacteria was observed. The significance of this phenomenon is discussed.