Serogroup prevalence of Shigellae in Bombay.

M Sonawala, K Saraswathi, LP Deodhar 
 Dept of Microbiology, LTM Medical College, Sion, Bombay.

Correspondence Address:
M Sonawala
Dept of Microbiology, LTM Medical College, Sion, Bombay.

Full Text

::   Introduction

Dysentery has been one of the major epidemic diseases in the history of mankind and one of the commonest killing diseases in the Pediatric age group all over the world. Outbreaks of Shigellosis have been reported from different parts of India[1],[2]. However, reports regarding prevalence of various serogroups of Shigella in Bombay are very few. The present investigation was therefore undertaken  (i) to study the incidence and serogroup prevalence of Shigella isolated from cases of dysentery during a three year period. (June 1988May 199l): (ii) to determine their drug resistance pattern, and (iii) to compare the results of the present study with that of previous years as regards the prevalent serogroup and antimicrobial sensitivity pattern of Shigella isolates.

::   Methods

A total of 2.758 faecal specimens collected from Pediatric (< 12 yrs) patients with diarrhoea/dysentery were studied for enteric pathogens. Samples of faeces were collected in sterile wide mouth bottles and rectal swabs were transported in CaryBiair transport medium and were processed within 2 hours of collection. The samples were examined macroscopically for the presence of mucus and blood and microscopically for pus cells, RBCs, macrophages cysts and ova of parasites.

Bacteriological examination

The samples were inoculated directly on the MacConkey agar, deoxycholate citrate agar (DCA) and enrichment was done in Selenite F broth and incubated at 37?C overnight. After the enrichment. subcultures were done on above mentioned culture media and further incubated at 37?C overnight. All the 2,758 samples were also processed for other enter-opathogens using standard laboratory procedures[3]. For Yersinia species, samples were incubated on Yersinia selective agar at 2025?C overnight. For Campylobacter jejuni CampyBAP medium was used and samples were incubated under microaerophilic conditions at 37?C - 42?C overnight.

Colonies morphologically resembling Shigella species were further identified by biochemical reactions and confirmed by slide agglutination test using polyvalent and monovalent antisera from Burroughs Wellcome, England.

Antibiotic Sensitivity tests: Drug susceptibility patterns of all Shigella isolates were determined by the disc diffusion technique of Kirby Bauer[4] Commercially manufactured discs (BioPasteur) of antimicrobial agents and their concentration in mcg/ml were as follows: gentamicin (G)  10, kanamycin (K)  30, ampicillin (A)  10, chloramphenicol (C)  30, streptomycin (S)  10, tetracycline (T) ( 30. nalidixic acid (NA)  30 and cotrimoxazole (CoT)  25. A control strain of E coli (ATCC  225922) was included in each test. The minimum inhibitory concentration (MIC) of A, S, C, T and CoT was determined using agar dilution methods[5].

::   Results

Various enteropathogens isolated during the present study are shown in [Table:1].

Isolation rate of Shigella species was 3.2% and was almost similar to that of Salmonella species (3.4%). Of 90 Shigella strains isolated, 66 were of Shigella flexneri (73.3%) followed by Shigella dysenteriae 1 (16.6%), Shigella boydi (8.8%), and Shigella sonnei (1.1 %). [Table:2] shows the serogroup distribution of these 90 isolates.

Shigella flexneri was the predominant serogroup isolated and maximum number of Shigella was isolated during June 1989 to May 1990. Among Shigella group B, Shigella flexneri 2a was more common (83%) followed by Shigella flexneri 4a (15%) and Shigella flexneri 6 (2%). Eighty per cent of the Shigella isolated were from the children below the age of 5 and remaining 20% from 512 yrs of age.

Comparison of antimicrobial sensitivity pattern of Shigella isolated from 1983 to 1991 is shown in [Table:3].

In the present study 100% sensitivity was seen with nalidixic acid followed by gentamicin (98.8%) and kanamycin (77.7%). The MIC of different drugs did not vary much from species to species or between strains. MIC of resistant Shigella isolated from 198891 is shown in [Table:4].

In 1983  84, a similar study[6] was carried out in the same institute and the predominant serogroup was Shigella dysenteriae I (78%) followed by Shigella flexneri (21.9%). Shigella boydii and Shigella sonnei strains were not detected during that period. The isolates were sensitive to Gentamicin. kanamycin, Ampicillin and Cotrimoxasole. From 1985 onwards (unpublished observations) a change in the serogroup prevalence was observed and majority of Shigella isolates belonged to Shigella flexneri (75%) followed by Shigella dysenteriae I (18%) till the year 1987. The resistance pattern observed in Shigella dysenteriae I strains isolated during the present study and those isolated in previous years was found to be similar. However, marked increase in the resistance in Shigella flexneri strains was seen with Ampicillin and Cotrimoxazole compared to the study carried out in previous years.

::   Discussion

Shigella still account for a significant proportion of bacillary dysentery in many tropical and subtropical countries[7].

A total of 90 Shigella were isolated from 2758 samples processed giving an isolation rate of 3.2%. Shigella flexneri accounted for the majority of the total Shigella isolated in the present study (73.3%) followed by Shigella dysenteriae I (1.6%). Our findings of serogroup prevalence among Shigella species are similar to other investigators[8],[9],[10].

Shigellosis was more common in first five years of life as 80% of Shigella were isolated from this age group. Same are the findings of other workers[11],[12].

All Shigella isolated were sensitive to nalidixic acid. Most of the isolates of Shigella dysenteriae I and Shigella flexneri were resistant to multiple antibiotics. The common resistance pattern observed were ACST and ACSTCoT Similar resistance pattern has been observed by Dudeja et al[13], and Shahid et al[14].

The results of the present study were compared with that of our previous publications[6]. Shigella dysenteriae I was the predominant serogroup isolated in previous years. However, 1985 onwards. Shigella flexneri predominated. In the present findings also, Shigella flexneri has dominated over other Shigella serogroups. In the previous study the incidence of Shigellosis was 2.0 to 2.5% which has increased to 3.2% during 19881991. This may be attributed to an outbreak of Shigellosis due to multi-drug resistant strains of Shigella flexneri.

Comparison of the sensitivity pattern of Shigella isolated during previous years and now has revealed that the change in pattern of Shigella serogroup is also associated with an increase in the resistance to Ampicillin and cotrimoxazole. Our findings arid those reported by Arora et al[1] show an increase in drug resistance with Ampicillin, chloramphenicol. streptomycin and tetracycline as against those reported by Kaliyug et al[9] and Panicker et al[15].

MIC studies have confirmed high level of drug resistance to commonly used antibiotics. The results of the MIC of resistant strains are comparable with that of Panicker et al[15] and Macaden et al[16]. However lower values have been reported to these antibiotics previously by Kaliyuga et al[9] and Gaikwad and Deodhar.

The present study indicates the emergence of resistant strains of Shigella flexneri to cotrimoxazole which is the drug of choice in the treatment of Shigellosis. An increased resistance to cotrimoxazole may be attributed to an indiscriminate use of this drug.

References