Journal of Applied Microbiology 1999, 87, 402–409 Study of the potential relationship between the morphology of infectious somatic coliphages and their persistence in the environment M. Muniesa, F. Lucena and J. Jofre Departament de Microbiologia, Facultat de Biologia, Universitat de Barcelona, Barcelona, Spain 7089/02/99: received 23 February 1999, revised 5 May 1999 and accepted 13 May 1999 M. MUNIESA, F. LUCENA AND J. JOFRE. 1999. The proportions of different morphological types of infectious somatic coliphages were determined in faecally polluted freshwaters. Myoviridae, followed by Siphoviridae, were the most frequently isolated morphological types in raw sewage, treated sewage and river water collected a few metres downstream from a sewage outfall. However, in river water collected further downstream from the pollution point, in river water after ‘in situ’ inactivation experiments and in chlorinated raw and treated sewage significant changes in the proportions of the different somatic coliphage morphological types occurred. In all cases, Siphoviridae, especially those with flexible and curled tails, became more abundant to the detriment of Myoviridae. INTRODUCTION Somatic coliphages (Hilton and Stotzky 1973; Wenstel et al. 1982; IAWPRC 1991), F-specific RNA bacteriophages (Hav- elaar and Hogeboom 1983; IAWPRC 1991) and Bacteroides fragilis bacteriophages (Jofre et al. 1986; Tartera and Jofre 1987; IAWPRC 1991) have been proposed as model organ- isms in water quality studies. Measuring somatic coliphages is easier, cheaper and faster than measuring the other groups of bacteriophages. However, they present some short- comings, one of them being the heterogeneity of the group, whereas both F-specific RNA (Havelaar and Hogeboom 1983) and B. fragilis (Jofre et al. 1986) bacteriophages are much more homogeneous groups. Indeed, the host strain Esch- erichia coli C (ATCC 13076), recommended in Standard Methods for the Examination of Water and Wastewater (Anon. 1992), or its nalidixic acid-resistant mutants WG5 (Grabow and Coubrough 1986) or CN13 (Payment and Franco 1993), recommended by ISO (Anon. 1998) for the detection of somatic coliphages, detect a set of different bacteriophages which, among other characteristics, differ in morphology (Ackermann and Nguyen 1983; Pedroso and Martins 1995). Moreover, in water samples collected far from the pollution source or in water samples taken after chemical disinfection, the predominant bacteriophages may be different from the Correspondence to: Dr J. Jofre, Departament de Microbiologia, Universitat de Barcelona, Avinguda Diagonal 645. 08028 Barcelona, Spain (e-mail: joan@porthos.bio.ub.es). © 1999 The Society for Applied Microbiology predominant phages detected in sewage and in recently pol- luted waters (Dee and Fogleman 1992; Lasobras et al. 1997). Further knowledge on this occurrence is necessary to better evaluate the potential usefulness of somatic coliphages as indicators of the hygienic quality of water or as tools to monitor water treatment processes. Here we compare the predominant morphological types of infectious somatic coliphages present in sewage, in receiving waters sampled at sites distant from the pollution point, in river water after ‘in situ’ inactivation experiments and in both raw and treated sewage after chlorination. MATERIALS AND METHODS Bacterial host and media Escherichia coli CN13 (Payment and Franco 1993), grown on tryptone medium (10 g tryptone, 1 g glucose and 5 g NaCl in a final volume of 1 l distilled water) containing nalidixic acid (100 mg l −1 ), was used for the detection of somatic coliphages. To study the effect of the assay medium on the distribution pattern of morphological types, modified Scholten’s medium (10 g peptone, 3 g yeast extract, 12 g meat extract, 3 g NaCl, 0·7 g Na 2 CO 3 and 1·25 mmol magnesium in 1 l distilled water) was also used (Havelaar and Hogeboom 1983). All the results, except those in Table 7, were obtained using tryptone soft agar as the assay medium.