J Bacteriol Virol.  2009 Dec;39(4):287-294. 10.4167/jbv.2009.39.4.287.

Different Responses of MSSA and MRSA to Oxacillin of Their Respective MICs

• Jung JS
• Shin WS
• Kim SK
• Park YS

• KMID: 2168540
• DOI: http://doi.org/10.4167/jbv.2009.39.4.287

Abstract

All of the methicillin-resistant Staphylococcus aureus (MRSA) strains exhibit resistance to oxacillin by producing PBP2a encoded by mecA, whereas methicllin-susceptible Staphylococcus aureus (MSSA) strains do not. To investigate phenotypic differences other than oxacillin resistance level in responses to oxacillin between MSSA and MRSA, we compared alterations of viability and ultrastructure of MSSA by oxacillin treatment with those of MRSA. When MSSA and MRSA strains were exposed to oxacillin of their respective MICs, and then were assayed for viability and observed by transmission electron microscope, increase in thickness of cell wall was more prominent in MRSA strains than in MSSA strains, while decrease in number of surviving cells was more evident and change in morphology of growing cross wall was greater in MSSA strains than in MRSA strains. It is assumed that these different responses to oxacillin between MSSA and MRSA strains may be due to activation of some PBP2a unbound to oxacillin. In conclusion, MSSA and MRSA showed different functional and morphological responses to oxacillin, although they were treated with oxacillin of concentrations that respectively inhibit their proliferation.

Keyword

MSSA; MRSA; Oxacillin; PBP2a

MeSH Terms

Adenosine
Cell Wall
Electrons
Methicillin-Resistant Staphylococcus aureus
Oxacillin
Staphylococcus aureus
Adenosine
Oxacillin

Figure

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  Figure 1. Growth curves of MSSA and MRSA strains.

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  Figure 2. Viability of MSSA and MRSA against oxacillin. Oxacillin of their respective MICs (MSSA, 0.5 μg/ml; MRSA, 1024 μg/ml) was added at 4.5 h (arrow).

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  Figure 3. Electron micrographs of MSSA grown in the absence (A, B) or presence (C, D) of oxacillin of its MIC (0.5 μg/ml) for 180 min. Bars = 260 nm.

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  Figure 4. Electron micrographs of MRSA grown in the absence (A, B) or presence (C, D) of oxacilln of its MIC (1024 μg/ml) for 180 min. Bars = 260 nm.

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  Figure 5. Schematic diagram of alteration in cell wall and cross wall of MSSA and MRSA treated with oxacillin of their respective MICs. A, C, E, and G are control cells, and B, D, F and H are oxacillin-treated cells. cW, cross wall; pW, peripheral cell wall; Sp, splitting system.

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