title: The effects of antibiotic stress on the expression of virulence factors by strains of Staphylococcus aureus displaying vancomycin-intermediate resistance
Creators: name: Everett, Lucy Margaret
Publisher: name: University of Glasgow
Year: 2003
Subjects: subject: Pharmacology, Staphylococcus aureus, anti-infective agents.

description: In this study, we examined the investigational protein synthesis inhibitors linezolid and quinupristin/dalfopristin, and daptomycin, a disrupter of cell membrane potential, along with cefpirome, a cell wall inhibitor, against strains of S.aureus displaying vancomycin-intermediate resistance (VISA). Six VISA strains were chosen from three countries, Japan (Mu3 and Mu50), United Kingdom (3700w and 3759v) and the United States (5827 and 5836) along with an epidemic strain of MRSA, EMRSA-16, and a standard methicillin sensitive S.aureus (MSSA) Cowan. Sub-inhibitory concentrations of cefpirome, daptomycin, linezolid and synercid were used to investigate their effects on cell growth and morphology, virulence factor expression, and strain susceptibility to opsonophagocytosis. Characteristics of individual strains in the absence of antimicrobial stress were compared to standard strains EMRSA16 and MSSA in addition to antimicrobial treatment being compared to the antibiotic free control for individual strains. It was found that each strain of VISA has distinctive characteristics and that the vancomycin resistant genotype does not confer the same phenotypic characteristics on every strain. A feature observed by other researchers and confirmed here, was the existence of much thickened cell walls in selected VISA strains. The lack of surface protein A was also apparent by its presence on VISA 5827 only in addition to the MSSA and EMRSA-16 strains. Extracellular protein A was also not uniformly detected but was expressed by four of the VISA strains. Exposure to sub-inhibitory concentrations of all four of the antimicrobial agents was generally found to reduce the expression of the surface proteins, clumping factor and protein A, in the majority of strains but produced variable responses in their effects on alpha-haemolysin and toxic shock syndrome toxin 1 (TSST-1) expression. Analysis of mRNA transcription of the protein A gene, spa, revealed increased levels of mRNA in response to antimicrobial stress however, the levels of mature protein were reduced confirming that linezolid and quinupristin/dalfopristin are inhibitors of protein synthesis. With regard to opsonophagocytosis, through microscopic visualisation, each strain appeared to be ingested to similar degrees by polymorphonuclear cells (PMNL) with the exception of Mu50 which was ingested to significantly higher degrees following exposure to cefpirome, linezolid and synercid. Using chemiluminescence as an indicator of phagocytic ingestion strain 3759v induced significantly less chemiluminescence in comparison to all of the other strains, including the standards, whereas, Mu3 and Mu50 induced greatly enhanced results. Exposure of the cell cultures to any of the antimicrobial agents did not have a uniform effect on opsonophagocytosis of the strains but chemiluminesence was found to be reduced in strains where antimicrobial agents exerted an effect. Protein A, amongst others, is regarded as an antiphagocytic protein. The absence of protein A on the surface of the majority of VISA strains was not found to increase the susceptibility of strains to opsonophagocytosis. Several strains possessed thickened cell walls that may be compensatory for the lack of protein A and affect phagocytosis as expression of thickened cell walls in response to antimicrobial stress reduced the level of chemiluminescence observed in several of the strains. Thus, the effects of different antimicrobial agents on a bacterial cell are not necessarily those exerted upon all bacterial strains. The characteristics assayed for here only represent a small fraction of a wider picture of the interaction between bacteria and antimicrobial agent. (Abstract shortened by ProQuest.).