Byline: X. Zhou, Y. Dai, X. Ma
The development of reduced vancomycin susceptibility in Staphylococcus aureus in many cases appears to be associated with characteristic changes. These changes may have pitfall of identifying S. aureus by automated testing methods like Vitek 32. In this study, we retested 24 heterogeneous vancomycin-intermediate Staphylococcus haemolyticus (h-VISH) collected in 2008-2010 at the Department of Clinical Microbiology by conventional biochemical tests and polymerase chain reaction (PCR). The heterogeneous vancomycin-intermediate S. aureus (hVISA) reversion test and electron microscopic examination were also used. Six isolates of 24 h-VISH possessed nuc, coa, and 16S rRNA genes, and could be reversed into S. aureus. It suggested that biochemical and morphological changes in hVISA and vancomycin-intermediate S. aureus (VISA) should be considered, and the detection of S. aureus, especially reduced vancomycin susceptibility isolates, requires more attention and different techniques.
Staphylococcus aureus is one of the most common causative pathogenic bacteria of suppurative skin inflammation, osteomyelitis, meningitis, sepsis and other infections. [sup] The high mortality due to S. aureus was decreased by penicillin in the 1940s, but this was short lived by acquisition of antimicrobial resistance in S. aureus. Methicillin-resistant S. aureus (MRSA) that are cross resistant to all [sz]-lactams, including penicillins and cephalosporins, account for hospital- and community-acquired infections worldwide. The glycopeptide antibiotic, vancomycin, was first introduced in 1958 as the most effective treatment for MRSA infections. Since first being reported in 1997, [sup] several cases of vancomycin-intermediate S. aureus (VISA) and heterogeneous vancomycin-intermediate S. aureus (hVISA) infections have been reported around the world.
VISA and hVISA usually develop changes in morphology and biological characteristics, such as cell wall thickening, slow growth, smaller colonies, decreased pigment formation, less or not haemolytic, reduced coagulase activity, etc. [sup] Coagulase, a major phenotypic determinant of S. aureus, is often used to identify S. aureus. [sup] If the coagulase test is inaccurate, automated testing methods like Vitek 32 (bioMerieux) may have pitfall as Staphylococcus haemolyticus and S. aureus possess similar biochemical tests.
Materials and Methods
A total of 24 h-VISH strains were used in this study by population analysis profiles. These strains were collected in 2008-2010 from clinical specimens including sputum (n=10, 41.7%), prostatic fluid (n=9, 37.5%), blood (n=2, 8.3%), wound secretion (n=1, 4.2%), pus (n=1, 4.2%) and drainage fluids (n=1, 4.2%). The isolate was plated on fresh blood agar and was incubated overnight at 37[degrees]C in 5% CO [sub]2 . The colony was identified by coagulase test, catalase test, and further confirmed by Vitek 32. In the tube coagulase test, the organism was mixed with 0.5 ml of rabbit plasma in a test tube and incubated at 37[degrees]C for more than 4 h and then at room temperature overnight. Tubes were examined for clot formation at 30-min intervals until 24 h....