Staph with a zone of inhibition.

Q In my work as a clinical microbiologist, I recently came across something I had not seen before. On a vaginal culture from a three-year-old girl in the ER, I found a Staphylococcus colony, which had created a zone of inhibition where alpha Streptococcus (normal flora) could not grow. I subcultured both organisms out of curiosity, and the zone of inhibition phenomenon was repeated. The Vitek Jr. that I use gave me an identification of Staphylococcus sciuri. I was unable to identify the alpha strep, and neither was the reference lab to which I sent a subculture. I am wondering what the S sciuri might have produced that could have spread into the agar and prevented the growth of alpha strep. It did not inhibit any of the other staph normal flora.

A A study published by Saigh, et al, in 1978 showed that endocervical flora (specifically streptococci, staphylococci, and lactobacilli) could inhibit the growth of Neisseria gonorrhoeae in vitro, while a study by Bisaillon, et al, in 1980 showed similar inhibition of N. gonorrhoeae growth by isolates of coagulase-negative staphylococci and group D streptococci. (1,2)

A more recent study indicated that strains of streptococci (specifically S. mutans, S sanguis, and S 'mitior') could inhibit the growth of staphylococci strains due to the production of bacteriocins. (3) Bacteriocins are proteinaceous toxins given off by bacteria that inhibit the growth of similar bacterial strain(s). According to Jack, et al (4), bacteriocins are "ribosomally synthesized peptides of 30 to less than 60 amino acids, with a narrow to wide antibacterial spectrum against Gram-positive bacteria; the antibacterial property is heat stable, and a producer strain displays a degree of specific self-protection against its own antibacterial peptide. They appear to be translated as inactive prepeptides containing an N-terminal leader sequence and a C-terminal propeptide component. Following post-translational modification, and depending on the pH, the molecules may either be released into the environment or remain bound to the cell wall. The antibacterial action is produced principally by destabilization of membrane functions." Staphylococcins (bacteriocins produced by staphylococci) have been studied from both human and nonhuman animals. Several studies are cited for further reading. (5,6,7,8,9) Staphylococcins show a wide range of inhibition against Gram-positive and Gram-negative organisms from different sources (Dr. Robert Sautter, personal communication). (5) It is possible that the coagulase-negative staphylococci isolated in this genital culture produced a bacteriocin(s), which was able to inhibit the growth of the alpha-streptococci in the culture and upon subculture testing.

References

1. Saigh JH, Sanders CC, Sanders WE Jr. Inhibition of Neisseria gonorrhoeae by aerobic and facultatively anaerobic components of the endocervical flora: evidence for a protective effect against infection. Infect Immun. February 1978;19(2):704-710.

2. Bisaillon JG, Beaudet R, Saheb SA, Morisset R. Interference of Neisseria gonorrhoeae growth by aerobic bacterial representatives of the urogenital flora. Rev Can Biol. December 1980;39(4):201-208.

3. Tzannetis SE, Bigis A, Konidaris N, loannidis H, Genimatas V, Papavassiliou J. In-vitro bacteriocin-mediated antagonism by oral streptococci against human carrier strains of staphylococci. J Appl Bacteriol. April 1991;70(4):294-301.

4. Jack RW, Tagg JR, Ray B. Bacteriocins of gram-positive bacteria. Microbiol Rev. June 1995;59(2):171-200.

5. Laukova A, Marekova M. Antimicrobial spectrum of bacteriocin-like substances produced by rumen staphylococci. Folia Microbiol(Praha). 1993;38(1):74-76.

6. Raczynska A. Bacteriocinogenic properties of Staphylococci isolated from the oral cavity. Med Dosw Mikrobiol. 1989;41(2):73-80.

7. Balusek J, Hajek V. Antagonistic activities of coagulase-positive staphylococci. J Hyg Epidemiol Microbiol Immunol. 1985;29(2):147-154.

8. Nakamura T, Yamazaki N, Taniguchi H, Fujimura S. Production, purification, and properties of a bacteriocin from Staphylococcus aureus isolated from saliva. Infect Immun. February 1983;39(2):609-614.

9. Rogolsky M, Wiley BB. Production and properties of a staphylococcin genetically controlled by the staphylococcal plasmid for exfoliative toxin synthesis. Infect Immun. March 1977;15(3):726-732.

--Susan E. Sharp, PhD (DABMM)

Director of Microbiology Kaiser Permanente; Associate Professor Pathology Regional Laboratory Oregon Health and Science University Portland, OR