Culture conditions affect cytotoxin production by Serratia marcescens

Some like it hot: the effects of climate change on reproduction, immune function and disease resistance in the cricket Gryllus texensis

In many parts of the world, climate change is increasing the frequency and severity of heat waves. How do heat waves impact short-lived poikilotherms such as insects? In the cricket, Gryllus texensis, 6 days of elevated temperatures (i.e. 7°C above the average field temperature and 5°C above their preferred temperature) resulted in increased egg laying, faster egg development and greater mass gain. The increased temperature also increased activity of phenoloxidase and lysozyme-like enzymes, two immune-related enzymes, and enhanced resistance to the Gram-negative bacterium Serratia marcescens. When given a sublethal S. marcescens infection, G. texensis maintained increased reproductive output at the elevated temperature (33°C). These data suggest that heat waves could result in more numerous, disease resistant, crickets. However, resistance to the Gram-positive bacterium, Bacillus cereus was lower at temperatures above or below the average field temperature (26°C). A sublethal infection with B. cereus reduced egg laying at all temperatures and suppressed the increase in egg laying induced by higher temperatures. These results suggest that for some species-pathogen interactions, increased temperatures can induce trade-offs between reproduction and disease resistance. This result may partly explain why G. texensis prefers temperatures lower than those that produce maximal reproductive output and enhanced immune function.

Proteolytic activity in Serratia marcescens clinical isolates

Exoproteinase production was demonstrated in 64 clinical isolates of S. marcescens. A significant relationship was found between the site of origin (autopsy material, hemocultures, various other sources), proteinase activity, and LD50 of the analyzed isolates. The number of exoproteinases varied during a 14-h incubation in batch cultures; the most frequently found was a 57.5-kDa proteinase which was observed in all analyzed strains. The exoproteinase production was shown to be related to strain virulence.

Biological activity of Serratia marcescens cytotoxin

Serratia marcescens cytotoxin was purified to homogeneity by ion-exchange chromatography on a DEAE Sepharose Fast Flow column, followed by gel filtration chromatography on a Sephadex G100 column. The molecular mass of the cytotoxin was estimated to be about 50 kDa. Some biological properties of the cytotoxin were analyzed and compared with well-characterized toxins, such as VT1, VT2 and CNF from Escherichia coli and hemolysin produced by S. marcescens. The sensitivity of the cell lines CHO, HeLa, HEp-2, Vero, BHK-21, MA 104 and J774 to the cytotoxin was determined by the cell viability assay using neutral red. CHO and HEp-2 were highly sensitive, with massive cellular death after 1 h of treatment, followed by BHK-21, HeLa, Vero and J774 cells, while MA 104 was insensitive to the toxin. Cytotoxin induced morphological changes such as cell rounding with cytoplasmic retraction and nuclear compactation which were evident 15 min after the addition of cytotoxin. The cytotoxic assays show that 15 min of treatment with the cytotoxin induced irreversible intoxication of the cells, determined by loss of cell viability. Concentrations of 2 CD50 (0.56 g/ml) of purified cytotoxin did not present any hemolytic activity, showing that the cytotoxin is distinct from S. marcescens hemolysin. Antisera prepared against S. marcescens cytotoxin did not neutralize the cytotoxic activity of VT1, VT2 or CNF toxin, indicating that these toxins do not share antigenic determinants with cytotoxin. Moreover, we did not detect gene sequences for any of these toxins in S. marcescens by PCR assay. These results suggest that S. marcescens cytotoxin is not related to any of these toxins from E. coli.

Characterization of a cytotoxic factor in culture filtrates of Serratia marcescens

Serratia marcescens culture filtrates have been reported to be cytotoxic to mammalian cells. Using biochemical and genetic approaches, we have identified a major source of this cytotoxic activity. Both heat and protease treatments abrogated the cytotoxicity of S. marcescens culture filtrates towards HeLa cells, suggesting the involvement of one or more protein factors. A screen for in vitro cytotoxic activity revealed that S. marcescens mutant strains that are deficient in production of a 56-kDa metalloprotease are significantly less cytotoxic to mammalian cells. Cytotoxicity was significantly reduced when culture filtrates prepared from wild-type strains were pretreated with either EDTA or 1,10-phenanthroline, which are potent inhibitors of the 56-kDa metalloprotease. Furthermore, cytotoxic activity was restored when the same culture filtrates were incubated with zinc divalent cations, which are essential for enzymatic activity of the 56-kDa metalloprotease. Finally, recombinant expression of the S. marcescens 56-kDa metalloprotease conferred a cytotoxic phenotype on the culture filtrates of a nonpathogenic Escherichia coli strain. Collectively, these data suggest that the 56-kDa metalloprotease contributes significantly to the in vitro cytotoxic activity commonly observed in S. marcescens culture filtrates.

Comparsion of selected growth media for culturing Serratia marcescens, Aeromonas sp., and Pseudomonas aeruginosa as pathogens of adult Stomoxys calcitrans (Diptera: Muscidae)

Stable flies, Stomoxys calcitrans (L.), were orally infected with Aeromonas sp., Pseudomonas aeruginosa (Schroeter), and Serratia marcescens Bizio that were cultured on egg-yolk media, nutrient broth, and fly egg media. Aeromonas and Serratia caused mortality when the bacteria were originally grown on egg-yolk medium. Pseudomonas was equally lethal regardless of the media on which it was cultured. A wild isolate of Aeromonas caused greater death than an isolate that had been passed through host flies and had been reisolated from killed flies. Mortality increased with bacterial dose for all species. Aeromonas and Serratia caused mortality within several days after ingestion, whereas Pseudomonas caused a gradual increase in mortality 3-7 d after ingestion. The pathologic activity of Aeromonas and Serratia required extracellular products produced when cells were grown in egg yolk medium. Aeromonas required both supernatant and cells from egg yolk medium, wereas Serratia required supernatant from egg yolk medium and cells from either nutrient broth or egg yolk medium. Mortality due to ingestion of Aeromonas was correlated with the presence of enzymes that cause alpha- and beta-hemolysis, while mortality following ingestion of Serratia was associated with alpha-hemolysins, elastases, and chitinases.

Effects of carbon and nitrogen sources, sodium chloride and culture conditions on cytotoxin production by Salmonella choleraesuis

The effects of carbon and nitrogen sources, incubation temperature, shaking speed, initial pH of culture broth as well as various concentration of NaCl on the production of cytotoxin by Salmonella choleraesuis SC-5 were evaluated in the present study. Results reveal that the optimal temperature, initial medium pH and shaking speed for cytotoxin production was 37 degrees C, pH 6.0-8.0 and 100 rpm, respectively. Tryptone was the best of the eight nitrogen sources tested for toxin production by S. choleraesuis. Among the nine carbon sources tested, S. choleraesuis produced a higher amount of cytotoxin in media containing glucose, fructose, galactose, sorbitol or mannitol as the carbon source. No toxin was detected in broths containing 4.0% or more sodium chloride in Tryptic soy broth (TSB). Cultures of S. choleraesuis in the medium containing 2.0% tryptone, 0.5% NaCl, 0.25% K2HPO4 and 0.25% of the best carbon source under the optimal conditions for 14 h resulted in the highest cytotoxin production. The Vero cell CD50 of S. choleraesuis lysate of cells grown under these optimal conditions was a titer of 589-758 per mg of lysate protein.