Escherichia coli and the protective role of
Lactobacillus casei in newborn rabbits
Esther van Praag, Ph.D.
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Newborn rabbits are unique in the animal world as their gastrointestinal tract is sterile and remains vastly devoid of bacteria during the first weeks of their life (depending on the studies it varies between a few days up to 3 weeks), even though the pH of the doe’s milk ranges between 5 and 6.5, which would allow bacterial survival, growth, and colonization. Rabbit milk, indeed, is rich in C8 and C10 fatty acids that have bacteriostatic properties and inhibits growth of bacteria in the digestive tract, of the newborn. These molecules are sometimes referred to as "milk oil", and their activation needs two parameters, milk of the mother and stomach of the suckling. This protects day-old newborn against the growth of pathogen bacteria and to die of necrotizing enteritis or generalized sepsis. Bacteria that are absorbed at the time of birth and via the milk will survive the passage through the stomach and intestine and will accumulate in the cecum and the final section of the digestive system (colon) and rectum.
At the age of 10-15 days, newborn rabbits become strong, inquisitive and leave the nest. They start nibbling solid food (hay, fresh vegetables, pellets) as well as the hard feces of the doe. Up to the age of 6 weeks, young rabbits gradually decrease milk intake. As a result:
- The concentration of C8 and C10 fatty acids (milk oil) decreases gradually in the anterior digestive system;
- The pH of the stomach drops from about 5-6 down to 1-2, becoming a barrier for colonizing bacteria and killing ingested bacteria;
- Microorganisms that were ingested previously and survived in the cecum and lower portion of the intestine and rectum will start to multiply. Translocation (spread) of intestinal bacteria from the rectum up to the upper portion of the small intestine (duodenum) becomes possible. When the right bacteria are present, proper digestion and fermentation of solid food becomes possible. This process takes a few days to weeks.
The different stages of bacterial colonization of the digestive tract are very important, but also very delicate. Indeed, microorganisms that develop inside the GI tract depend on the type of diet, hay, stress. Colonization of yet sterile portions of the intestine by pathogen bacteria may lead to organ failure, necrotizing enteritis, diarrhea and (fatal) enteric trouble or sepsis.
It is therefore important to keep the rabbits with the mother up to the age of 8 weeks, and not separate them to early, unless needed. Once a healthy bacterial flora has colonized the GI tract, the development of pathogen bacteria will be hindered. It is speculated that food fermented by lactobacilli help keep homeostasis between Lactobacilli sp., and other microbial flora of the GI tract, in animal species that host this bacteria naturally.
Orphaned rabbit babies fed on alternative source of milk don’t get the protective C8 and C10 fatty acids, and often show increased sensitivity to enteric infections. One of the bacteria that cause fatal enteritis is Escherichia coli. A recent paper (2001, see ref) discusses the protective role of Lactobacillus casei - a bacterium found in probiotic preparation such as Protexin, Probiocin, Benebac or Probios, against a toxin producing strain of E. coli.
This particular strain has been shown to cause hemorrhagic colitis (inflammation of the colon, accompanied by heavy bleeding), hemolytic-uremic syndrome (fever, acute renal failure, dissolution of red blood cells, and low number of platelet cells), and complications in the central nervous system. The mucosal damage in the GI tract is severe though not equal, and correlates with the number of observed pathogenic bacteria, and the concentration of the toxin: more pronounced in the cecum, and colon, less in the small intestine.
The study showed that most of the infected rabbits developed diarrhea quickly. Newborn rabbits treated with L. casei developed diarrhea too, but symptoms were less severe (16% severe diarrhea against 77.3% in the control group). The growth of the bacteria did not show a difference in both groups up to day 4 after infection; at day 7, the number of viable bacteria was 100 fold lower in the L. casei treated group. The same is observed for the toxin concentration: it remained stable after day 4 in L. casei treated rabbits.
Histopathologic examination of cecum, intestine, and colon showed that the control group suffered severe damage:
Small intestine: - necrosis,
- vacuolation of epithelial cells.
Cecum: - exfoliation of epithelial cells
- pseudo-eosinophil (type of white blood cell) infiltration,
- mitotic activity.
Colon: - exfoliation of epithelial cells necrosis
None of the above pathology has been observed in L. casei treated rabbits.
The pH of the stomach and intestine are high in newborn rabbits, 5.1 and 6.5, respectively, and no differences were observed between the 2 groups. (Those high values probably help the L. casei bacterium survive the gastric passage, and enable it to colonize the colon, cecum and large intestine of young rabbits.) The concentration of lactic acid was slightly higher in L. casei treated rabbits, as compared to the control group.
Interestingly, volatile fatty acids, like lactic acid, are known for their potent bactericidal activities, and are often added to the diet or to the water of weaning animals, in order to reduce the development of pathogenic bacteria.
Lactic acid furthermore acts on the membrane of cells, affecting the rate of exchange of H+ and Na+ ions, the activity of the plasma membrane H+/ATPase activities, and the fatty acid composition of the membrane. This leads to an acidification of the cytoplasm. In the case of this particular E. coli strain, it was shown that a concentration of 3.2 mM lactic acid was needed to inhibit its development, while less 1 mM was found in vivo. The presence of lactic acid thus may not explain the differences observed between the control, and L. casei treated group.
Lactobacilli and Bifidobacilli are, moreover, known to increase the secretion of IgA (immunoglobulin A or antibody that deactivates the activity of foreign bodies). This was observed in the newborn rabbits too. Higher levels of toxin present in a particular organ (i.e. colon) correlated to a higher the level of mucosal IgA present.
This study indicates that preventive administration of L. casei to newborn rabbits protects them from E. coli enteritis, by enhancing the secretion of specific IgA. Protection by its production of lactic acid is doubted in this case, since the in-vitro bactericidal needed 3 times higher concentrations.
Thanks are due to Linda Bayley (USA) and the rescued doe, and to Karen Comish (Israel) and Pepe feeding her newborn baby.
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