Rakhshanda Baqai  ( PMRC Research Centre, Jinnah Postgraduate Medical Centre, Karachi- 35. )

Diarrhoeal disease is caused by a variety of pathogens which include bacteria, parasites and viruses. Apart from the major aetiological agents (Salmonella, Shigella, pathogenic E. Coli) Aero­monas is also a significant enteric pathogen¹. Its infection should always be looked for before con­sidering the culture as negative, because Aero­monas is not a part of normal human faecal flora² or opportunist pathogen³. Aeromonas is widely distributed in nature and can be pathogenic to man and animal. It is a facultative anaerobic, gram negative rod shaped bacteria, first discovered in the last decade of 19th century as natural inhabitant of aquatic environ­ment and animals. Aeromonas has a cosmopolitan distribution in marine and freshwater, sewage, soil and food⁴. Survival seems to depend on moisture and the presence of organic matter⁵. Aeromonas have been recovered from tap water³, hospital water⁶, swimming pooi, lakes⁷ and rivers. Human infection is often related to water exposure^5,8 and drinking of untreated water is risky for susceptable hosts^9,10. Studies have shown simultaneous isolation from faeces and water sup­ply. Although several characteristics differ in clini­cal and environmental strains but many of the strains found in water have properties, identical with those of clinical isolates and they may be potential enteric pathogens^11,12. Majority of iso­lates from human source are recovered from gast­rointestinal and intraabdominal sites and are transient in nature resulting in self limiting and lo­calized infection^13,14. Animal and human carriers are also present¹⁵. Aeromonas may act as primary pathogen occurring during spring and summer and in children under two years of age. There have been reports of acute diarrhoea associated with Aeromonas hydrophilia^16,17 but the virulence fac­tor varies from one country to another¹⁸. The dif­ference in isolation may be due to geographic location, seasonal difference and method of isola­tion^19,20. Duration of illness is greater than 10 days with fever, abdominal cramps and vomiting²¹. Vol­unteer studies or intestinal biopsies of patients with diarrhoea may be required toestablish Aero­monas as gastrointestinal pathogen²². Aeromonas has been associated with a wide range of human infection, the site of infection and the host defence status defining the severity of disease. Positive cultures have been reported from bile in biliary obstruction, Cholecystitis²³, and Se­pticaemia mostly in association with hepatobiliary disease, malignancy⁵, intraabdominal abscess and surgical wound infections²⁴, urinary tract infec­tion, rare cases of meningitis, peritonitis, otitis and endocarditis¹. Cellulitis or wound infection is re­lated to exposure to water or soil and occur most frequently during warm season²⁵. Aeromonas grows well on moist enteric sel­ective medium. For isolation from faeces sheep blood agar added with ampicillin (15 ug per ml) can be used²¹. Overnight enrichment in alkaline peptone water yield 2-6 times more isolates as compared to other media like MacConkey agar and SS agar²⁶. A. bydrophilia in faeces is detected by flooding the plate with oxidase reagent after growth. A positive oxidase test is an important biochemical properly of this genus. A. hydrophilia are hemolytic²⁷. Identification can be done by API-20E²⁸. Biochemical characters associated wi­th virulence and tests used for Aeromonas are positive Voges-Proskauer reaction, production of gas from glucose, fermentation of mannose and absence of B D glactosidase and esculin hydro­lisis²⁹. Antibodies to A. hydrophilia (antihemo­lysin, agglutinin, precipitin) have been detected in patients with deep infection. Several toxic factors have been isolated from A. hydrophilia culture flu­terates (leucocidin, hemolysin, cytotoxin). A hydrophilia is resistant to Ampicillin and Carbencillin with variable susceptibility to Cep­halosporins, Polymixin and Tetracyclines. It is us­ually susceptible to aminoglycosides, Naladixic acid Nitrofurontoin, Chloramphenicol and Sep. tran^30,31. Although there are reports regarding the presence of Aeromonas in diarrhoeal cases in In­dia⁸, Thailand^32,33 and Bangladesh, it is possible that Aeromonas is present in diarrhoeal patients in Pakistan, therefore a preliminary study is pla­nned at PMRC Research Centre, Karachi to de­tect this neglected pathogen.

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