Journal of Applied Microbiology 1999, 86, 169–173 Note: Susceptibility to chlorine of Aeromonas hydrophila strains S. Massa 1 , R. Armuzzi 2 , M. Tosques 1 , F. Canganella 3 and L.D. Trovatelli 3 1 Istituto di Produzioni e Preparazioni Alimentari, Facolta ` di Agraria, Foggia, 2 Romagna Acque, Capaccio, S. Sofia, Forlı ` , and 3 Dipartimento di Agrobiologia e Agrochimica, Universita ` della Tuscia, Viterbo, Italy 6657/04/98: received 3 April 1998, revised 20 May 1998 and accepted 1 June 1998 S. MASSA, R. ARMUZZI, M. TOSQUES, F. CANGANELLA AND L.D. TROVATELLI. 1998. The susceptibility of five Aeromonas hydrophila strains and one Escherichia coli strain to chlorine was studied under carefully controlled laboratory conditions. Of the Aer. hydrophila strains, two were from untreated water, two from tap water (immediately downstream of a water treatment plant) and one from the DSM collection. The study included disinfectant concentration (0·1, 0·2 and 0·5 mg l −1 ), pH (6, 7 and 8) and temperature (4, 21 and 32 °C) as controlled variables. The results indicated that the untreated water strains, the DSM strain and the E. coli strain were inactivated within 1 min of chlorine treatment. The strains from chlorinated water (TW11 and TW27) showed a different susceptibility to chlorine disinfection, the rate of inactivation being greater at pH 6 than at pH 8 for both strains. Under the standard conditions of temperature 21 °C, pH 7 and chlorine concentration 0·2 mg l −1 , an increase or decrease of approximately 1 log unit in the number of bacteria did not affect the kill rate of the strains TW11 and TW27. INTRODUCTION Aeromonas hydrophila was isolated from a wide range of foods (Abyeta and Wekell 1988; Fricker and Tompset 1989) and from most waters, even chlorinated drinking water (Hazen et al. 1978; Burke et al. 1984b; Van der Kooij 1988). While its role in wound infections and sepsis in immunocompromised patients has been well documented for several decades, the significance of Aer. hydrophila as the causative agent of human food- and water-borne disease is a matter of controversy. Gastroenteritis linked with, but not necessarily caused by, Aer. hydrophila strains has been reported (Gracey et al. 1982; Altewegg and Geiss 1989). Such strains usually possess viru- lence factors (i.e. ability to produce enterotoxin, cytotoxin, haemolysin and proteases) which are recognized in other pathogenic bacteria (Cahill 1990). Human feeding studies have been inconclusive and only two of 57 healthy volunteers developed diarrhoea following ingestion of organisms from diarrhoeal faeces (Morgan et al. 1985). However, in this trial no data were given regarding the adhesive properties of the Correspondence to: Dr Salvatore Massa, Istituto di Produzioni e Preparazioni Alimentari, Facolta ` di Agraria, via Napoli, 25–71100 Foggia, Italy. © 1999 The Society for Applied Microbiology strains used or the immune status of the adult volunteers. So, with present knowledge, it cannot be excluded that enter- opathogenic Aer. hydrophila strains, when present in suf- ficiently large numbers, may infect susceptible hosts including children and elderly persons. Despite the occurrence of this bacterium in chlorinated water supplies, it is believed to be susceptible to free chlorine. Burke et al. (1984a) found that an increase in the Aeromonas count was related to a decrease in the free chlorine levels. Similarly, Van der Koij and Hijnen (1988) concluded that the frequent isolation of aeromonads from chlorinated water supplies may have been due to the inability to maintain a residual concentration of chlorine necessary to eliminate these micro-organisms from water. Also, Krovacek et al. (1992) found Aer. hydrophila in chlorinated drinking water from two different municipal distribution systems in Sweden through- out the study period. There have been surprisingly few studies on the inac- tivation of Aer. hydrophila by chlorine (Cattabani 1986; Kno chel 1991; O ¨ zbas and Aytac ¸ 1994). Therefore, the behav- iour of various strains of Aer. hydrophila after exposure to a range of chlorine concentrations equivalent to that found in tap water in most distribution systems was studied.