Appl Microbiol Biotechnol (2003) 61:424–428 DOI 10.1007/s00253-003-1302-y MINI-REVIEW S. Sharma · P. Sachdeva · J. S. Virdi Emerging water-borne pathogens Received: 1 October 2002 / Revised: 27 February 2003 / Accepted: 28 February 2003 / Published online: 9 April 2003  Springer-Verlag 2003 Abstract Emerging water-borne pathogens constitute a major health hazard in both developed and developing nations. A new dimension to the global epidemiology of cholera—an ancient scourge—was provided by the emergence of Vibrio cholerae O139. Also, water-borne enterohaemorrhagic Escherichia coli (E. coli O157:H7), although regarded as a problem of the industrialized west, has recently caused outbreaks in Africa. Outbreaks of chlorine-resistant Cryptosporidium have motivated water authorities to reassess the adequacy of current water- quality regulations. Of late, a host of other organisms, such as hepatitis viruses (including hepatitis E virus), Campylobacter jejuni, microsporidia, cyclospora, Yersin- ia enterocolitica, calciviruses and environmental bacteria like Mycobacterium spp, aeromonads, Legionella pneu- mophila and multidrug-resistant Pseudomonas aerugi- nosa have been associated with water-borne illnesses. This review critically examines the potential of these as emerging water-borne pathogens. It also examines the possible reasons, such as an increase in the number of immunocompromised individuals, urbanization and hor- izontal gene transfer, that may underlie their emergence. Further, measures required to face the challenge posed by these pathogens are also discussed. Emerging water-borne pathogens In 1992, the Institute of Medicine described an emerging infection as any new, re-emerging, or drug-resistant infection whose incidence in humans has increased within the past two decades or whose incidence threatens to increase in the near future (Lederberg et al. 1992). A number of reviews and other publications available on the subject discuss emerging and re-emerging pathogens in general, including water-borne, without addressing the issues specifically pertinent to emerging water-borne pathogens. This review critically examines which organ- isms really qualify as emerging water-borne pathogens, the possible reasons underlying their emergence and specific measures to face the challenge posed by them. Due to the paucity of data, it may be difficult to decide which of all water-borne pathogens are emerging. Nev- ertheless, there are some clear-cut candidates. No organism other than Vibrio cholerae could serve as a better example of an emerging water-borne pathogen. Cholera is an ancient scourge and to date seven pandemics have been recorded. Of the several recognized serogroups, V. cholerae O1 has been responsible for these pandemics. In 1992 however, a new strain called V. cholerae O139 Bengal appeared in South India and caused explosive outbreaks of cholera-like disease (Ra- mamurthy et al. 1993). In a matter of one year, the new strain spread to several parts of India and to neighbouring Bangladesh and Thailand (Nair et al. 1994). By the end of 1993, cholera outbreaks due to V. cholerae O139 were reported from South Asia and other countries of the world. Soon after its appearance, V. cholerae O139 outnumbered V. cholerae O1 and became the dominant serogroup in India and other parts of the Indian sub- continent. It was thought that this was probably the beginning of a new pandemic—the eighth pandemic of cholera (Nair et al. 1996). But, by 1994, there was a dramatic decrease in V. cholerae O139 and once again V. cholerae O1 became the dominant species. It is thought that, in the years to come, V. cholerae O139 is going to play an important role in the global epidemiology of cholera (Garg et al. 1998). Following identification of the O139 serogroup and the finding that environmental, non-toxigenic strains may play an important role in the evolution of toxigenic V. cholerae (Karaolis et al. 1995), there has been a lot of interest in the study of non-O1, non-O139 serogroups. At least three localized outbreaks of diarrhoea caused by S. Sharma · P. Sachdeva · J. S. Virdi ( ) ) Microbial Pathogenicity Laboratory, Department of Microbiology, University of Delhi South Campus, Benito Juarez Road, 110 021 New Delhi, India e-mail: virdi_dusc@rediffmail.com Tel.: +91-11-26879950 Fax: +91-11-26885270