Septicemia with two distinct strains of Staphylococcus aureus and dwarf variants of both

A patient with erythema multiforme developed septicemia with two strains of Staphylococcus aureus. Six blood cultures, obtained during 14 days, yielded a mixture of both strains. The strains differed in the ability to hemolyze human erythrocytes, in the production of β-lactamase and in the susceptibility to antibiotics and bacteriophages. Following therapy with gentamicin, a Dwarf colony variant auxotrophic for vitamin K, tryptophan and p-aminobenzoic acid appeared in some of the blood cultures. This variant was apparently derived from one of the offending strains (phage Type 55/71). From other blood cultures, a different type of Dwarf colony variant was obtained; this variant was of the same phage type as the other offending S. aureus strain (Type 85). The metabolic deficiencies of this strain were not unraveled. Both events, the double infection with distinct S. aureus strains, and the emergence of Dwarf variants during antibiotic therapy, are unusual in clinical practice, and will be detected only if laboratory staff pays particular attention to the possible occurrence of such events.