Background:Staphylococcus aureus, with reduced vancomycin susceptibility, is probably under the regulation of several genes and various express phenotypes.Objectives:This study aimed to investigate the phenotypic differences between vancomycin-susceptible S. aureus (VSSA), vancomycin-intermediate S. aureus (VISA), and heterogeneous VISA (hVISA) isolates.Materials and Methods:A total of 130 methicillin-resistant S. aureus (MRSA) isolates were studied, including 49 VSSA, 28 hVISA, and 5 VISA isolates from blood cultures and 48 isolates (two VSSA, six hVISA, and 40 VISA) derived in vitro (laboratory-induced/sub-passaged). Their phenotypes were examined using a coagulase tube test, colony spreading on soft agar, and urease activity. The SCCmec and agr typing were performed using multiplex PCR.Results:Most of the MRSA isolates were SCCmec III-agr I (84.5%), followed by SCCmec II-agr II (11.8%). The average plasma coagulation time of vancomycin-non-susceptible isolates was longer than that of the susceptible isolates (12 vs. 2.6 hours). Four hVISA (P = 0.023) and nine VISA (P < 0.001) isolates yielded a negative coagulase test after 24-hour incubation. The percentage of VSSA isolates showing non-spreading colonies (accessory gene regulator (agr) dysfunction) was significantly lower than in the VISA group (P = 0.013), but no significant difference was found between VSSA and hVISA. The VISA group showed higher urease activity than that of the VSSA and hVISA groups (P = 0.002).Conclusions:There were diverse phenotypic changes among vancomycin-non-susceptible S. aureus isolates. This may be due to the variety of related regulatory systems. The diversity of phenotypic expression may result in its misidentification in routine laboratory checks.

Staphylococcus aureus is the most common cause of nosocomial fevers. We investigated the virulence genes and genotypes of S. aureus strains isolated from bloodstream infections of patients in a Thai teaching hospital. Thirtythree methicillin-resistant S. aureus (MRSA) and 52 methicillin-susceptible S. aureus (MSSA) collected during 1997-1998, and 29 MRSA and 52 MSSA strains collected during 2010-2011 were studied. Susceptibility to 8 antimicrobials was determined using an agar dilution method. Twelve virulence genes were detected by polymerase chain reaction. The bacterial strains were typed by SCCmec, agr, spa, and multilocus sequence typing. The majority of the MSSA isolates were susceptible to almost all antimicrobials tested, whereas the MRSA isolates were resistant to more than 3 of the antimicrobials tested. The hla-sea was the most common virulence gene profile in the MRSA isolates from both periods (46% in 1997-1998, 31% in 2010-2011), and hla alone was the most common pattern in the MSSA isolates (56% in 1997-1998, 35% in 2010-2011). The S. aureus isolates in 2010-2011 contained more virulence gene profiles than those in 1997-1998. All MRSA isolates from 1997-1998 carried SCCmec III-agr I, whereas those in 2010-2011 carried SCCmec III-agr I (48%) and SCCmec type II-agr II (31%). No specific virulence genes or genotypes of the isolates related to a poor clinical outcome were found.