Colonial Morphology of Staphylococci on Memphis Agar: Phase Variation of Slime Production, Resistance to -Lactam Antibiotics, and Virulence

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Abstract
The growth of Staphylococcus epidermidis sensu stricto and Staphylococcus saprophyticus on Memphis agar yielded up to 6 morphotypes with each strain. With S. epidermidis, one morphotype produced slime (ρ) but became non-slime-producing (ε) at a high frequency. The slime-producing ρ variants were methicillin-resistant and more virulent than methicillin-susceptible ε variants in an endocarditis model. With S. saprophyticus, phase variation was of higher frequency. Nitrosoguanidine mutagenesis produced a stable blue ε form that wasmore virulent than the parent in a mouse model of urinary tract infection. Mutants with the blue ε phenotype differed from gold ε parents in a variety of phenotypic properties, including increased resistance to oxacillin. These staphylococcal species have a high frequency of phase variation: Phase variants differ in antibiotic resistance and virulence, which is only partially correlated with suggested virulence factors such as slime production.