Evaluation of blood culture media for isolation of pyridoxal-dependent Streptococcus mitior (mitis)

Abstract

Nutritional variant streptococci identified as pyridoxal-dependent Streptococcus mitior (mitis) account for 5 to 6% of streptococcal endocarditis and may be a cause of "culture-negative" endocarditis. Hence, growth of three variant strains in 11 commercial blood culture broths was compared to that in fresh heart infusion broth. For simulation of clinical specimens, culture bottles were injected with 5 ml of human blood, inoculated with approximately 500 colony-forming units (CFU) per bottle, and monitored for 7 days with Gram stains and viable counts. Only Thiol broth (Difco Laboratories, Detroit, Mich.) supported growth without blood at this low inoculum. In media containing blood, maximal growth of 10(9) CFU/ml was reached within 2 days of incubation, and heavy turbidity was consistently observed in only heart infusion broth, Thiol broth, and media supplemented with pyridoxal hydrochloride. Columbia broth (BBL Microbiology Systems, Cockeysville, Md.) with increased cysteine, thioglycollate broth, and one brain heart infusion broth produced moderate growth (1 x 10(8) to 5 x 10(8) CFU/ml), whereas Columbia broth, another brain heart infusion broth, and two brands of tryptic soy broth showed fair growth (1 x 10(7) to 4 x 10(7) CFU/ml). The poor growth (1 x 10(6) to 3 x 10(6) CFU/ml) observed in three other brands of tryptic soy broth was often not apparent macroscopically or by Gram stain. Furthermore, on growth occurred in 40% of tryptic soy broth cultures inoculated with 50 CFU. Therefore, to ensure isolation of these variant streptococci from clinical blood cultures, a medium containing thiol compounds or supplemented with pyridoxal should be used. Subcultures should be made within 2 days of incubation to blood agar enriched with pyridoxal or containing a Staphylococcus sp. streak for satellitism.