Prevalence and Characteristic of Aeromonas Species and Plesiomonas Shigelloides in Lagos.
This study was carried out in two phases. The first phase involved the determination of the prevalence of Aeromonas species. and Plesiomonas Shigelloides in patients, with and without diarrhoea, reporting at the enteric laboratory of the Department of Medical Microbiology and Parasitology, Lagos University Teaching Hospital (LUTH), Idi-Arba, Lagos: over a 12-month period of October 1986 through September 1987. In the second phase the characteristics of isolates thus obtained were examined. During the first phase of the study, a total of 2,350 faecal specimens from patients with diarrhoea were screened for the presence of Aeromonas sp. or P. Shigelloides. Of this number, 53 (2.26%) yielded P. Shigelloides sp. On the other hand, of a total of 500 faecal specimens from patients without diarrhoea (controls) collected over the same period of time, only 2 (0.4%) were positive for Aeromonas sp. while none of the specimens yield P. shigelloides. In which case, both organisms were more frequently isolated from diarrhoea than that from control specimens. As a matter of fact, the difference in the isolation rates of Aeromonas sp. from patients with diarrhoea and controls was statistically significant (p<0.01). It is equally noteworthy that isolation rates of 2.26% Aeromonas sp. and 0.68% P. shigelloides compared favourably with those obtained for other established enteric pathogens in this study, such as the entero-pathogenic Escherichia coli (EPEC, 2.85%), shigella sp. (0.17%), Salmonella sp. (0.13%) and Y. enterocolitica (0.09%). Also, none of these enteropathogens were recovered in faecal specimens positive for Aeromonas sp. or P. shigelloides. Further biochemical characterisation of the Aeromonas and Plesiomonas isolates revealed that of the 55 Aeromonas sp. 20 (36.4%) were A. Hydrophila, 28 (50.9%) were A. caviae and 7 (12.7%) were A. Sobria. It was also observed that of the numerous tests used in the characterisation, eleven simple ones, viz: the production of oxidase, catalase, hydrogen sulphide (H2S) from Kligler's iron agar (KTA), pigmentation on nutrient agar, acid from glucose, xylose, dulcitol, adonitol, mannitol, inositol, and inability to grow in 6.5% NaC1; were highly reproducible. Based on these findings, a simple scheme was designed for the isolation and preliminary identification of Aeromonas sp. and P. shigelloides from faecal specimens. Similarly, survival of Aeromonas sp. and P. shigelloides was measured in commonly used transport media such as Cary-Blair, Stuart, Amies, phosphate bufferred saline, and alkaline peptone water (APW). Results obtained indicate that these organisms would survive in these media for at least one month. However, a modified form of APW containing 0.01% w/v FeSo4 appeared to be the most suitable for the long storage of these organisms. In the same vein, a good number of Aeromonas sp. and P. shigelloides examined in this study produced haemolysins, heat-stable (ST) enterotoxin, or invasiveness: which are recognised virulence factors in the pathophysiology of acute diarrhoea. Specifically, about 70% of the Aeromonas sp. produced & or B-haemolysin, 20% produced ST and about 11% were invasive. Similarly, 25% of P. shigelloides produced ST and invasiveness while about 13% produced & - haemolysin. It is significant to note that all the isolates that produced virulence factors were isolated from patients with diarrhoea; thus indicating that these factors may have a role to play in diarrhoea associated with these organisms. However, when strains producing virulence factors were fed orally to healthy adult mice, they failed to cause diarrhoea. It is noteworthy that these strains survived the gastric acid barrier in the stomach, and were infact reisolated from faecal specimens of the animals. The possibilities are therefore that either these virulence factors were not elaborated in vivo, or that a number of host factors made mice unsuitable for experimental infection with Aeromonas or Plesiomonas. The in vitro susceptibility pattern of Aeromonas sp. and P. shigelloides isolated in this study was determined against 15 selected antimicrobial agents. The results obtained have shown that these organisms are susceptible to most of the agents evaluated. Specifically, all the strains were susceptible to ofloxacin and at least 60 (>70%) strains were susceptible to gentamicin, carbenicillin, ceftriaxone, colistin, nalidixic acid, nitrofurantoin, chloramphenicol, tetracycline and co-trimoxazole. Also, less than 30% of the strains were susceptible to ampicillim. Finally, Aeromonas sp. and P. shigelloides isolated during the course of this work were screened for the presence of plasmids. Interestingly enough, some of the isolates harboured plasmids with molecular weights ranging between 2.4 x 106 to 16.8 x 106 daltons. Of the 55 Aeromonas sp. examined, 12 (21.8%) harboured one or more plasmids. Similarly, a strain of P. shigelloides possessed more than one plasmid. Hence, the various findings reported in this study seem to suggest that Aeromonas sp. and P. shigelloides may have a significant role to play in acute diarrhoeal diseases in the Lagos environment. However, there is need for similar studies on a national scale to increase the present level of awareness; and for a better understanding of Aeromonas and Plesiomonas associated diarrhoea in Nigeria.