Virulence of Escherichia coli Serotypes for Mice

Correlation between pathogenicity of Shigella and intraperitoneal survival in mice

Phagocytosis of virulent and avirulent strains of Shigella and Escherichia coli in the mouse peritoneum was studied. A direct correlation between bacterial virulence and resistance to phagocytosis by peritoneal phagocytes was demonstrated. Virulent strains were less readily cleared and were able to multiply to a limited extent within the peritoneal cavity. An epimerase-deficient, rough mutant of S. flexneri 2a was highly susceptible to phagocytosis. Restoration of the cell wall structure in these mutants resulted in a significant increase in their resistance to phagocytosis. Susceptibility to phagocytosis in smooth S. flexneri was age-dependent. Cells from 16-hr cultures were more resistant to removal from the peritoneum than were cells from 48- and 72-hr cultures.

Cytotoxins in non-enterotoxigenic strains of Escherichia coli isolated from feces of diarrheic calves

We have examined the cytotoxic responses produced in HeLa and Vero cell cultures by sonicates from 15 non-enterotoxigenic (STa-, LT-) strains of E. coli, highly lethal for mice parenterally LD50 less than 3 X 10(7) CFU), which had been isolated from feces of diarrheic calves. Three types of cytotoxic responses were observed. Type 1 (five strains) consisted of enlargement, rounding and polynucleation of HeLa cells, an effect previously reported with cytotoxic necrotizing factor (CNF) in E. coli from infant and piglet enteritis. Type 2 toxicity (three strains and the control Vir strain S5) was also characterized by enlargement and polynucleation of HeLa cells, but in contrast to Type 2 effect, cells were elongated. Sonicates from the latter strains were lethal for chickens, producing the lesions previously described with Vir strains. Type 3 toxicity (two strains and the control VT strain H19), produced an extensive destruction of both Vero and HeLa cell cultures. Cytotoxic effects were completely abolished upon heating for 1 h at 60 degrees C for Type 1 and 2 extracts and at 80 degrees C for Type 3 extracts. Seroneutralization assays showed that cytotoxins of the same type were closely related antigenically. In addition, a slight cross-neutralization was observed between Type 1 (CNF) and Type 2 (Vir) toxins.

A footpad lesion test to demonstrate the pathogenicity of Escherichia coli strains

A virulence test based on the ability of E. coli strains to cause lesion in the footpads of mice was devised to demonstrate their pathogenicity. Virulence was expressed by a scale from 0 to 7 according to the importance of the lesions. Among 54 strains tested, those inducing death after intraperitoneal inoculation caused severe oedema or necrosis (score greater than 4) in footpads. Three other virulence tests (virulence measured in pregnant mice, in adult mice intraperitoneally challenged and in baby mice after oral inoculation) were used to study seven of the most characteristic strains. Good correlation was observed: strains pathogenic in the footpad lesion test were pathogenic in at least one of the other tests.

Nonspecific resistance to Escherichia coli in mice

Nonspecific cell-mediated immunity to a relatively virulent strain of Escherichia coli was studied in mice infected with Staphylococcus aureus and elicited with specific antigens. The infected and elicited mice were protected against as intraperitoneal challenge by E. coli for an observation period of 7 days, whereas normal mice, given the same number of bacteria, died within 18 to 24 h. However, the amount of time elapsing between elicitation and challenge greatly affected the rate of protection. Little or no protection was observed in mice injected with S. aureus but not elicited or in mice injected with staphylococcal antigens but not infected with staphylococci.

Mouse virulence of K(L) antigen-containing strains of Escherichia coli

The mouse virulence of two K antigen-containing (L variety) strains of Escherichia coli (serotype O2:K1) isolated from human septicemia, and of their variants which lacked K antigen, was studied. The strains containing envelope antigen (K+) were highly virulent when injected intracerebrally or when suspended in mucin and injected intraperitoneally. After intraperitoneal injection of E-107 K+ (but not K-), there was a marked initial growth in the peritoneal cavity followed by bacteremia and infection of all the organs examined. In the mucin-enhanced lethal infection, this growth continued until death of the animal; in the nonlethal infection, growth ceased and the count dropped quickly after approximately 5 hr. Host defenses were depressed greatly by intraperitoneally, but not intravenously, administered mucin. Bacteria were most virulent when injected intraperitoneally. In vitro phagocytosis of the K+ bacteria required opsonins not needed for phagocytosis of the smooth K- variants. Opsonins were found in immunized rabbit and normal mouse sera. Immune rabbit sera contained antibodies with anti-K specificity which were opsonic in vitro and highly protective in vivo when administered passively. There appears to be a lesser anti-O opsonic and protective activity involving one of the strains (E-107 K+), and colonial morphology, agglutination, and absorption tests indicated a low amount of K antigen on this organism. No anti-O opsonic or protective activity could be shown involving the other strain (E-102 K+). When standard serological typing procedures were used, these two strains appeared to be identical serologically, but they differed greatly in sensitivity to immune rabbit serum in phagocytosis experiments in vitro.

A bacteriological and serological study of experimental escherichia coli infection of calves

Twelve hundred bacterial cultures from thirty newborn calves were examined serologically to determine the experimental pathogenicity of E. coli serotypes as well as the influence of enteric bacteria naturally present in the calves. Serotype O115:K.:H18 was pathogenic, O9:K35:H1O was not, and unclassified E. coli OX28 was questionable. Klebsiella sp. and E. coli OX28 occurred naturally in the calves and was responsible for some mortality. Contact exposure in two groups of calves was not successful. All calves had a detectable gamma-globulin level whether or not they had nursed the dam. Antibody to four E. coli O groups was present at a gamma-globulin level above 0.78 gm% but not belw 0.60 gm%. Antibody was associated with survival of calves given E. coli serotype 115:K.:H18 but not the less virulent 9:K35:H1O. There was no correlation between antibody level and bactericidal activity of the serums. The latter did not appear to offer resistance to infection by itself, but antibody level and bactericidal activity to E. coli O115 and O78 did protect all calves experimentally infected with different serotypes.