Spheroplasts of Haemophilus influenzae induced by cell wall-active antibiotics and their effect upon the interpretation of susceptibility tests.

The interpretation of in vitro susceptibility tests of Haemophilus influenzae performed by the agar or broth dilution methods with Levinthal enrichment was found to be markedly influenced by the production of spheroplasts by this species. Using an inoculum of 10(7) organisms/ml, this phenomenon was frequently evident macroscopically as a haziness on agar substrates and in broths containing cell wall-acting agents, such as ampicillin, cephalothin, and penicillin, but was not noted with chloramphenicol. Phase-contrast microscopic examination of the haze from these sources revealed numerous spherical bodies in contrast to the typical cocco-bacillary forms observed in growth controls. With this inoculum size, minimal bactericidal concentrations could not be determined since subculture of 0.1 ml of the hazy broths or the surface haze onto chocolate agar resulted in most instances in the development of a small number of colonies which, upon smear and gram stain, revealed typical Haemophilus morphology. An inoculum of 10(4) organisms/ml abolished the haziness on agar surfaces and in broths and resulted in clear-cut end points. Also, although spherical bodies were still present, they were distinctly less in number as contrasted to tests performed with an inoculum of 10(7) organisms/ml. It is recommended that minimal inhibitory concentration end points in antibiotic susceptibility tests be determined by microscopic, rather than macroscopic, observation of the growth milieu to determine the presence or absence of morphologically typical bacilli which, when observed, is indicative of true in vitro resistance.