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Resistance of Salmonella serotypes to chloramphenicol * S. K. SETHI,' S. ANAND,2 AJAIB SINGH,3 & D. V. VADEHRA

Abstract A total of 704 Salmonella strains isolated from various sources were tested for their resistance to chloramphenicol. The results indicate that 36 out of 46 serotypes, comprising 671 strains (95.4 °O), were sensitive to chloramphenicol up to a level of 10 Mg per ml of medium. Most of the strains were sensitive at concentrations between 2 ug and 4 Mg of chloramphenicol per ml of medium. None of the strains isolated from animals, including poultry, was resistant. The results indicate that in India resistance of Salmonella strains to chloramphenicol has not increased over the years though this may have occurred in other parts of the world.

The WHO Expert Committee on Antibiotics recommended in its second report (1) that a survey of resistance of Salmonella typhi to chloramphenicol should be conducted in different parts of the world. Several workers in India (2-5) and elsewhere (8-12) have since obtained conflicting results regarding the frequency of chloramphenicol resistant Salmonella strains. In this paper we report results obtained with 704 Salmonella strains isolated from various sources and tested for their resistance to chloramphenicol.

Materials and methods Salmonella strains. The strains used in the present studies were identified as salmonellae at the National Salmonella and Escherichia Centre, Kasauli, India, by biochemical, serological, and culture methods (6). A total of 704 Salmonella strains belonging to 46 serotypes were tested for sensitivity to chloramphenicol. The strains were isolated from diseased and apparently healthy humans and animals (including poultry) and from meat, foods, sewage, water, and other sources. Most of the strains were isolated in different parts of India but some strains isolated

in Burma were sent to the Salmonella and Escherichia Centre for identification and serotyping. Antibiotic susceptibility test. Standard chloramphenicol powder was dissolved in fresh distilled water and diluted to a stock concentration of 100 ,tg/ml. A fresh solution of antibiotic was used for each experiment. Medium. The medium used was modified MullerHinton broth and Muller-Hinton agar described by Otaya et al. (7). Various volumes of antibiotic solution at the stock concentration were added to flasks containing 200 ml of agar medium that had been previously melted and cooled to 44°C in a water bath. The concentration of antibiotic varied from 0.5 ,ug per ml of agar medium. The medium was then poured into Petri dishes (20 ml per plate) and allowed to set at room temperature (20c-25oC). Each plate was divided into eight segments for inoculation of eight strains. Similar agar plates without antibiotic served as controls. Inoculation and reading of plates. Tubes containing 3 ml of broth were inoculated with a loopful of bacterial growth taken from a 24-h agar slant and incubated at 37'C for 18 hours. A standard inoculating wire loop was used to transfer a loopful of broth culture to a segment of an agar plate with and without added antibiotic. Inoculum for all strains was applied as spots of approximately equal size. The plates were incubated at 37^C for 18 hours. Organisms showing no growth were classified as sensitive while those showing confluent growth, as observed in the control plates, were considered resistant to the particular concentration of antibiotic incorporated in the medium. Results and discussion Reported resistance of Salnionella serotypes to chloramphenicol varies considerably. In India, Agarwal (2) reported that 9% of S. typhi strains were resistant to chloramphenicol up to a concentration of 16 jug/ml while Murti et al. (3) found that up to 20% of Salmonella tested were resistant. In Nigeria, Njoku-Obi & Njoku-Obi (13) reported chlor-

* From the Department of Microbiology, Panjab University, Chandigarh, India. Reader in Medical Microbiology.

'Research student. 3Lecturer in Pathogenic Bacteriology. ' Professor of Microbiology. 3527

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resistance. S. paratyphi A is relatively infrequent in India and only 1 out of 18 strains was resistant to chloramphenicol, while in S. paratyphi B, which is extremely rare in India, 3 out of 18 strains tested were resistant. All these strains were isolated in Burma. Maximum resistance (100%0) was observed in S. alachua strains isolated from water and sewage.

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ACKNOWLEDGEMENTS

The authors wish to thank the Director, Central Research Institute, Kasauli, India, for providing Salmonella strains. The chloramphenicol was kindly supplied by Parke-Davis (India) Ltd., Bombay.

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Fig. 1. Chloramphenicol sensitivity pattern in 704 Salmonella strains belonging to 46 serotypes.

amphenicol resistance in 25% of Salmonella serotypes. Our study shows that of 704 Salmonella strains, 671 (95.40 ) were sensitive to chloramphenicol at concentrations ranging from 0.5 ,ug to 10 pg per ml of medium. The sensitivity pattern for different concentrations of the antibiotic is shown in Fig. 1. Of the 46 serotypes tested, 36 were completely sensitive to chloramphenicol. Thirty-three strains that showed complete resistance included 29 clinical isolates from patients with symptoms of enteric fever, gasteroenteritis, and meningitis. The main serotypes manifesting these syndromes were S. typhi, S. paratyphi A, S. paratyphi B, S. virchow, S. newport, S. pulloruim, and S. enteritidis. The other four resistant strains, isolated from water and sewage, belonged to the serotypes S. alachua and S. london. These results indicate a much lower resistance of Salmonella serotypes to chloramphenicol than that found in some of the earlier studies (2, 3, 13) but are in agreement with those of other studies made in India (4, 5, 14), the Netherlands (15), Singapore (17), and the USA (16). S. typhi is the major serotype found in India, and in the present study comprised 23.9% of the 704 strains tested. Only 1.700 of these S. typhi strains showed chloramphenicol

1. WHO Technical Report Series, No. 210, 1961 (Standardization of methods for conducting microbic sensitivity tests: second report of the Expert Committee on Antibiotics). 2. AGARWAL, S.C. Bulletin of the World Health Organization, 27: 331-335 (1962). 3. MURTI, B. R. ET AL. Journal of clinical pathology, 15: 544-551 (1962). 4. SHARMA, K. B. & BHATIA, S. L. Indian journal of medical research, 54: 819-822 (1966) 5. NATH, M. L. & SINGH, J. Indian journal of medical research, 54: 217-219 (1966). 6. NATH, M. L. ET AL. Indian journal of medical research, 54: 512-516 (1966). 7. OTAYA, H. ET AL. Journal of antibiotics, 23: 324-335 (1970). 8. HUEY, C. R. & EDWARD, P. R. Proceeding of the Societyvfor Experimental Biology and Medicine, 97: 550-551 (1958). 9. MCWHORTER, A. C. ET AL. Applied microbiology, 11: 368-370 (1963). 10. YURACK, J. A. Canadian journal of microbiology, 10: 521-526 (1964). IL. ANDERSON, E. S. & DATTA, N. Lancet, 1: 407 (1965). 12. POCURULL, D. W. ET AL. Applied microbiology, 21: 358-362 (1971). 13. NJOKU-OBI, A. N. & NJOKU-OBI, J. C. Journal of bacteriology, 90: 552-553 (1965). 14. VAZE, S. & SHARMA, K. B. Indian journal of medical sciences, 25: 859-861 (1971). 15. MANTEN, A. ET AL. Tijdschrift voor diergeneeskunde, 88: 1858-1866 (1963). 16. ADAMS, R. ET AL. Applied microbiology, 16: 1570-1574 (1968). 17. SING, E. H. & LAM, S. Singapore medical journal, 12:8 (1971).

Resistance of Salmonella serotypes to chloramphenicol.

A total of 704 Salmonella strains isolated from various sources were tested for their resistance to chloramphenicol. The results indicate that 36 out ...
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