Inhibition of gonococci by a selective medium: disparity between isolates from sexual partners.

Abstract

CorrespondenceTO THE EDITOR, British Journal of Venereal Diseases Inhibition of gonococci by a selective medium: disparity between isolates from sexual partners Sir, A proportion of clinical isolates of Neisseria gonorrhoeae fail to grow on selective medium containing vancomycin, colistin, nystatin, and trimethoprim (VCNT). Reyn and Bentzon (1972) and Brorson et al. (1973) reported the isolation frequency of these strains to be about 4 % and 100% respectively and found vancomycin to be the inhibitory component. The sensitivity of these strains to vancomycin has been attributed to env mutations which result in phenotypic hypersensitivity to antibiotics including vancomycin (Sparling et al., 1976). Observations in this department suggest that other mechanisms may also be involved in the inhibition of certain gonococcal isolates by vancomycin.The isolation and identification of gonococci have been previously described (Platt, 1976a). Ready-poured bi-plates, containing GC selective medium (VCNT) and GC non-selective medium, were obtained from Gibco-Biocult (Paisley, Scotland) and each batch was quantitatively quality-controlled to ensure consistent inhibitory activity. The isolation frequency of VCNT-sensitive gonococci remained constant at between 4 % and 6 %.Gonococci were isolated from 32 patients attending the department of genitourinary medicine at the West London Hospital. From four (25 %) patients and their respective sexual partners gonococci were isolated which grew on non-selective medium but failed to grow on selective medium on both primary isolation and subsequent subculture. Gonococci isolated from a further 12 patients failed to grow on selective medium on primary isolation; of these six were male and six were female.On subculture each of five isolates tested produced colonies on selective medium which were similar in size and number to a control plate lacking VCNT. The gonococci isolated from each of the 12 sexual partners grew well on selective medium on primary isolation.Reyn and Bentzon (1972) found that about 40 % of strains, inhibited by vancomycin on primary isolation, were capable of growth on subculture if a heavy inoculum was used, and they attributed this result to random variation in the original inocula and a 'training effect' during laboratory subculture. The high proportion (75 %) of disparate vancomycin sensitivity between sexual partners on primary isolation and the resistance of the vancomycin-sensitive isolates on subsequent subculture suggests that an alternative mechanism is involved.Although env mutations have been demonstrated in clinical isolates of N. gonorrhoeae (Eisenstein and Sparling, 1978), for vancomycin sensitivity to be solely under env control would require both isolates from sexual partners to exhibit similar sensitivity to VCNT. The results obtained from four pairs of patients are compatible with this hypothesis. The disparate results obtained with 24 paired isolates may be explained as a function of the physiological state of the organism in vivo. For many organism...

Auxotypes and antibacterial resistance to gonococci with differing susceptibilities to vancomycin.

“…The progeny of isolated transformants were screened for altered drug responses and nutritional requirements. 24 strains exhibited nutritional requirements that included arginine, hypoxanthine, and uracil. The recovery percentages may have been affected by differences between the growthpromoting characteristics of the haemoglobincontaining basal medium used for VCNT and the GCMBS medium used for LC cultures.…”

“…[19][20][21][22][23][24][25][26][27] The observation that the percentage of vancomycin-hypersusceptible isolates increased over a two-year period in Denmark'5 necessitates continuing studies of hypersusceptible gonococci in other countries. If two primary culture media containing antibacterial agents are to be used, one medium should incorporate drugs which differ from vancomycin in their modes of action.…”

“…Nevertheless, the ability of marginally susceptible strains to form colonies on the primary culture medium is dependent on inoculum size'5 25-27 and on the physiological state of the bacteria. 24 When the diagnosis is not established patients with asymptomatic gonorrhoea may remain untreated, risking the development of complications. Moreover, a Danish 166 study'5 showed that continued transmission of vancomycin-hypersusceptible gonococci led to an increased percentage of such strains isolated in 1971 compared with 1969.…”