Salmonella regrowth in compost as influenced by substrate (salmonella regrowth in compost).

Composting can eliminate pathogenic organisms, including salmonellae, from sewage sludge. However, if salmonellae are present in the compost at undetectable levels or are inoculated into the compost by infected animals or from other sources, they may regrow presenting a health hazard for certain uses of compost. In this study, we examined dilute mineral-salt extracts of three composts from widely separate composting sites in the United States and found that they supported growth ofSalmonella typhimurium. From kinetic studies of the growth of the organism on these extracts, we concluded that each compost produced on extraction a single water-soluble substrate and that the substrates from the different composts were very similar, if not identical.

Microbially mediated growth suppression and death of salmonella in composted sewage sludge.

The role of compost microflora in the suppression of salmonella regrowth in composted sewage sludge was investigated. Microbial inhibition studies of salmonella growth were conducted on nutrient agar, in composts that had been subjected to different temperatures in compost piles, and in radiation sterilized composts inoculated with selected fractions of the compost microflora. Agar assays of inhibition indicated that bacteria and actinomycetes were not suppressive to salmonellae, but a few fungi were. However, compost inoculation assays showed consistently that fungi were not suppressive, but bacteria and actinomycetes were. In compost inoculation assays, microbial antagonists, when present, either killed salmonellae or reduced their growth rate. No suppression of salmonellae occurred in compost taken from 70°C compost-pile zones despite the presence and growth of many types of microbes. With greater numbers and kinds of microbes in 55°C compost, salmonella growth was suppressed 100-10,000-fold. Salmonellae died when inoculated into compost from unheated zones (25-40°C) of piles. Prior colonization of compost with only noncoliform gram-negative bacteria suppressed salmonellae growth 3,000-fold. Coliforms when inoculated prior to salmonellae accounted for 75% of salmonella die-off. Mesophilic curing to allow colonization of curing piles in their entirety by gram-negative bacteria, especially coliforms, should be an effective way to prevent repopulation by salmonellae.

Occurrence, growth, and suppression of salmonellae in composted sewage sludge.

Composted sewage sludge may be used to improve soil quality, but there remains some doubt concerning the microbiological safety of the product. Sewage sludge composts from 30 municipalities were sampled, and four samples (12%) contained salmonellae (two contained fewer than 0.3/g, and the other two had 21/g and 1.7 X 10(4)/g). All 30 composts were inoculated with salmonellae; the populations decreased at a specific death rate of about 0.15 h-1 over 24 h at 36 degrees C. In irradiation-sterilized composts inoculated with salmonellae, the salmonellae grew at a rate of 0.65 doublings per h for over 24 h. Growth and death rates were found to be moisture and flora associated. The growth or death rates for antibiotic-resistant salmonellae were not different from those of nonresistant strains. It was concluded that the active indigenous flora of compost establishes a homeostatic barrier to colonization by salmonellae, and in the absence of competing flora, reinoculated salmonellae may grow to potentially hazardous densities. The active microflora of moist composts eliminated contaminating salmonellae (10(5)/g) after 6 weeks.

Modified agar medium for detecting environmental salmonellae by the most-probable-number method.

Salmonellae in the environment remain a potential source of disease. Low numbers of salmonellae have been detected and enumerated from environmental samples by most-probable-number methods which require careful colony selection from a plated agar medium. A modified xylose lysine brilliant green medium was prepared to control the loss of selectivity caused by heating the brilliant green component. Added agar reduced colony spreading. The medium contained 47 g of xylose lysine agar base per liter; the agar content was adjusted to 2%, autoclaved, cooled to 50 degrees C, and then amended just before pouring to include H2S indicator and 7 ppm (7 ml of 1:1,000 brilliant green per liter) of unheated brilliant green dye. H2S-positive salmonellae were easily detected from sewage sludge compost to the exclusion of most other gram-negative bacteria. As a result, fewer non-salmonellae were picked for further most-probable-number analysis, greatly reducing the work load associated with the most-probable-number method. Direct plating was possible for enumerating salmonellae in laboratory composts containing ca. 10(3) or more salmonellae.

Effect of heat on virus inactivation by ammonia.

The rate of inactivation of bacteriophage f2 and poliovirus 1 (CHAT) by NH3 was strongly influenced by temperature. The process was pseudo-first order at all temperatures and NH3 concentrations. Poliovirus was inactivated at a greater rate than f2, but the change in the rate of inactivation with increasing temperature in the range of approximately 10 to 40 degrees C was greater for f2 than for poliovirus. At higher temperatures, the rate of change was greater for poliovirus. Arrhenius plots of the data were biphasic, indicating that two inactivation processes were occurring, one for the low temperature range and another for the high temperature range. However, the magnitudes of the thermodynamic variables for f2 were low enough, as calculated for the low (10 to 35 degrees C) and high (35 to 60 degrees C) phases, that inactivation could have occurred by breakage of nucleic acid chains. For poliovirus, the sizes indicated possible involvement of nucleic acid at the low temperatures (10 to 40 degrees C) but some unknown mechanism for the high temperatures (40 and 50 degrees C).

Kinetics of virus inactivation by ammonia.

Ammonia has been shown to be virucidal in sludge and NH(4)Cl solutions, although the rates at which viruses are inactivated have not been thoroughly studied. In the present studies, the kinetics of the poliovirus type 1 (strain CHAT) and bacteriophage f2 inactivation were examined in such a way that the effects of OH(-) and NH(4) (+) could be separated from those of NH(3). Purified virus stocks were placed into solutions of NH(4)Cl and control solutions containing an equivalent concentration of NaCl and incubated at 20 degrees C. The percentage of virus surviving was calculated, and the kinetics were evaluated by constructing semilogarithmic plots of data. At all pH values and NH(3) concentrations studied, the kinetics of the inactivation of both viruses were pseudo-first order. OH(-) had no measurable effect on the viruses, whereas the effects of NH(4) (+) and Na(+) were similar. A dose-response relationship between NH(3) and the viruses was also found. Bacteriophage f2 was approximately 4.5 times more resistant to the effects of NH(3) than was poliovirus.

Inactivation by ionizing radiation of Salmonella enteritidis serotype montevideo grown in composed sewage sludge.

S. enteritidis ser. montevideo were grown in composted sewage sludge to levels of approximately 10(9)/g. These bacteria were found to be inactivated by ionizing radiation at approximately the same rate (30 krads/log) as Salmonella species in liquid digested sludge.

Populations of dalapon-decomposing bacteria in soil as influenced by additions of dalapon or other carbon sources.

The numbers of bacteria capable of decomposing the herbicide dalapon were determined for five soils by the most-probable-number method. Before treatment of the soils with dalapon, the numbers varied from 1,000 to 10,000 cells per g of soil. Incubation of the soils with dalapon resulted in large increases (100-fold) in two of three soils in which dalapon was decomposed rapidly. Lack of increase in numbers in spite of rapid decomposition in the other soil probably indicated breakdown by a chemical process or decomposition by fungi. In the remaining two soils, in which decomposition was slow in one and did not occur in the other, the initial numbers were at the low end of the range and the increase was small on incubation with dalapon. Addition of ground alfalfa or ground corn plant material to a soil did not result in significant increases in the numbers of dalapon-decomposing bacteria, either during or after decomposition of the plant material. Glucose depressed the rate of breakdown of dalapon in the soil and increased dalapon-decomposing Bacillus species rather than Arthrobacter and Agrobacterium species, which were found to increase on incubation with dalapon itself. The most-probable-number method appears to be a valuable tool for pesticide-ecology studies.