Metabolic activity of bacterial cells enumerated by direct viable count.

The direct viable count (DVC) method was modified by incorporating radiolabeled substrates in microautoradiographic analyses to assess bacterial survival in controlled laboratory microcosms. The DVC method, which permits enumeration of culturable and nonculturable cells, discriminates those cells that are responsive to added nutrients but in which division is inhibited by the addition of nalidixic acid. The resulting elongated cells represent all viable cells; this includes those that are culturable on routine media and those that are not. Escherichia coli and Salmonella enteritidis were employed in the microcosm studies, and radiolabeled substrates included [methyl-3H]thymidine or [U-14C]glutamic acid. Samples taken at selected intervals during the survival experiments were examined by epifluorescence microscopy to enumerate cells by the DVC and acridine orange direct count methods, as well as by culture methods. Good correlation was obtained for cell-associated metabolic activity, measured by microautoradiography and substrate responsiveness (by the DVC method) at various stages of survival. Of the cells responsive to nutrients by the DVC method, ca. 90% were metabolically active by the microautoradiographic method. No significant difference was observed between DVC enumerations with or without added radiolabeled substrate.

Electrophoresis of DNA in ultrathin planar-format linear polyacrylamide.

Linear or un-cross-linked polyacrylamides have been employed successfully in the field of capillary electrophoresis for the separation of nucleic acids. Typical acrylamide concentrations for those applications range from 3% to 14% (wt/vol), with consistencies ranging from virtually liquid to moderately viscous. Due to the absence of cross-links, and the relatively fluid nature of linear polyacrylamide at typically employed concentrations, its use in planar (slab) gel electrophoresis has been overlooked. We describe herein the application of ultrathin (100 microns) high-viscosity slabs of linear polyacrylamide to planar electrophoresis of nucleic acid fragments. The approach we describe is rapid and yields high-resolution separations of nucleic acid fragments in linear polyacrylamide supports. The mobilities of DNA fragments of various lengths in a range of concentrations of linear polymer are compared with those observed for conventional cross-linked gels. The reptative migration of larger DNA fragments in linear polymers is predictable from the models derived from work with cross-linked acrylamide and agarose. The migration of smaller fragments, however, is not entirely predicted by the Ogston model. The relative mobilities observed for very small DNA fragments are approximately half those predicted by the Ogston regimen.

Viable but nonrecoverable stage of Salmonella enteritidis in aquatic systems.

An environmental isolate (13- 1BB ) of Salmonella enteritidis serogroup C1 was inoculated into sterile Potomac River water microcosms to observe survival and culturability of the organism by employing acridine orange direct count, fluorescent antibody direct count, direct viable count, plate count on veal infusion agar and xylose lysine decarboxylase agar, and indirect enumeration by the most-probable-number method (MPN), using media selective for Salmonella. Loss of culturability on laboratory media was observed within 48 h. However, cultures could be "resuscitated" and cultured on solid media, following addition of nutrients to the microcosms . Cells, resuscitated 4 days after apparent "die-off" (0 colony-forming units (cfu)/mL) using plate count techniques, yielded numbers of cfu in the same order of magnitude as had been observed before the onset of nutrient limitation. Microscopic techniques for direct viable counting indicated that viability is maintained for as long as 60 days after depletion of nutrients, although attempts to culture these cells, by addition of nutrient, after 21 days yielded apparently sterile plates. Thus, longer periods of "dormancy" appear to require conditions other than simple nutrient addition for resumption of cell growth and division.

The fate of enteric pathogenic bacteria in estuarine and marine environments.

Sufficient laboratory and field data are now available to hypothesize that enteric pathogens survive for very long periods of time in sea-water. In fact, these Gram-negative bacteria probably enter into dormancy, during which they remain viable and potentially virulent, yet are non-culturable when traditional bacteriological methods are employed. Increasing use of the world's oceans-for discharge of domestic wastes may result in public health problems in the future from the allochthonous human pathogens accumulating in the marine environment at disposal sites.