J Clin Pathol: first published as 10.1136/jcp.38.2.235 on 1 February 1985. Downloaded from

Letters to the Editor 235 Letters to the Editor Sensitivity of100 strains ofgroup A 3-haemolytic Room air 5% CO2 Anaerobic Choc BA Choc BA Choc BA of f3-haemolytic strep- No of strains sensidvity Sensitive 9 2 7 0 2 6 tococci group A Resistant 91 98 93 100 98 94 Optochin (ethyl hydrocuprein hydroch- Choc = chocolate agar. per- BA = blood agar. loride) sensitivity testing is routinely Sensitive = zone of inhibition >14 mm diameter. formed by most laboratories for the dif- Resistant = zone of inhibition < 14 mm diameter. ferentiation of Streptococcus pneumoniae from other streptococci. In our laboratory all streptococci, seen by Gram of blood cultures, are The zones of inhibition shown by group s-haemolytic streptococci and cannot sug- tested for sensitivity to optochin. The A /-haemolytic streptococci were compar- gest a mode of action for optochin. Kreger optochin test results are read after 4-6 h able to that of the Str pneumoniae con- et a16 showed that electron dense aggre- incubation at 36°C, along with other anti- trol. With the exception ol blood agar in gates appeared in the cytoplasm of treated biotic sensitivity tests. 5% CO2, zones of inhibition were shown cells, but this was found to be a non- This procedure was followed on a set of on both blood agar and chocolate agar specific effect. blood cultures from a patient who was sub- under different atmospheric conditions. Although we do not believe that there sequently shown to have a 83-haemolytic Four strains were sensitive to optochin are many laboratories that would mistake a streptococcus group A infection; however, only on blood agar in anaerobic conditions. ,3-haemolytic streptococcus group A for a there was a zone size >14 mm round the Another four strains showed sensitivity Strpneumoniae on an overnight culture, on optochin disc on the original chocolate agar only on chocolate agar in aerobic condi- rapid 4-6 h testing the S3-haemolysis may plate. On overnight testing 18-haemolysis tions. Other strains were sensitive to not be evident and another character, such was apparent, but the isolate still gave a optochin randomly on blood agar or choco- as microscopical appearance, must also be zone >14 mm on a fresh chocolate agar late agar or both in aerobic, CO2, or used before presumptive identifications are plate. anaerobic conditions. made. copyright. We have investigated the phenomenon There did not seem to be any correlation of optochin sensitive /8-haemolytic strep- between optochin sensitivity and the type GH BURGESS tococci group A and our results are pre- of medium used or the conditions of incu- J KHOR sented. bation. Department of Microbiology, None of the sensitive strains was bile St James Hospital, Material and methods soluble. London SW12

One hundred strains of group A Discussion http://jcp.bmj.com/ ,8-haemolytic streptococcus from various clinical sites were used. The isolates were Optochin sensitivity has been described for each streaked on to three sets of chocolate use in the identification of Str pneumoniae agar (Oxoid) and Columbia 5% horse by several workers.5. Although reports of References blood agar (Oxoid) plates. Two or three false sensitivity of the a-haemolytic strep- 'Lennette EH, Balows A, Hausler WJ, Truant colonies of each group A f3-haemolytic tococci do occur (<1 %), there have been JP, eds. Manual of Clinical Microbiology. streptococcus were streaked on to one no such reports with /8-haemolytic strains. 3rd ed. Washington DC: American Society quarter of the test medium and a 6 mm The largest zone size suggested by for Microbiology, 1980. on September 30, 2021 by guest. Protected optochin disc (Mast) was placed on the Austrian4-that is, 14 mm-was used to 2 Wilson GS, Miles AA. Topley and Wilson's innoculum. The plates were then incubated define a sensitive strain. Principles of bacteriology and immunology. at for hours under varying We have found that the incidence of 3rd ed. London: Edward Arnold, 1946. 36°C 18-24 3Bowers EF, Jeffries LR. Optochin in the conditions: (a) one set in a normal aerobic ,8-haemolytic strains sensitive to optochin identification of . J , (b) one set in a 5% CO2 is much higher than that reported for Clin Pathol 1955;8:58-60. incubator, and (c) one set in an anaerobic a-haemolytic streptococci. 4Austrian R. Diplococcus pneumoniae incubator (Don Whitley Scientific). The effect of a CO2 environment in (pneumonococcus). In: Blair JE, Lennette The original optochin sensitive group A reducing the zone size of some strains is EH, Truant JP, eds. Manual ofclinical mic- ,8-haemolytic streptococcus and a fresh also mimicked by Str pneumoniae. Rags- robiology. Bethesda, Md: American Society laboratory isolate of Str pneumoniae as a dale and Stanford5 described how the pres- for Microbiology, 1970. control were also included. ence of 5% CO2 diminished the zone size 5Ragsdale AR, Sanford JP. Interfering effect of After incubation zones of inhibition of Str pneumoniae to optochin and sug- incubation in carbon dioxide on the size > 14 mm was in identification of pneumonococci by optochin were measured. A zone gested that strains that do not grow discs. Appl Microbiol 1971; 22: 854-5. recorded as sensitive. All zone sizes atmospheric conditions should be tested by 6 Kreger AS, Swartzendruber DC, Olsen RH. < 14 mm were discounted.' the bile solubility technique. Alteration in bacterial morphology by The table records the sensitivity of the 100 We offer no explanation of the variation optochin and hydrochlorides. J Bac- strains tested. from strain to strain within the group A teriol 1969;97:362-6.