Effects of the Normal Flora on Host Defenses Against Microbial Invasion. In: Mirand EA, Back N, editors. Germ-Free Biology Experimental and Clinical Aspects. New York: Springer US; 1969. pp. 197-206. [DOI: 10.1007/978-1-4899-6495-3_27]

Role of intestinal anaerobic bacteria in colonization resistance

The purpose of this study was to clarify the role of the intestinal anaerobe bacteria in colonization resistance. Germfree mice were associated with Escherichia coli C25 and either (a) no other species; (b) enterococcus; (c) Escherichia coli M14 and Proteus mirabilis, or (d) Bacteroides fragilis and Bacteroides vulgatus. All species colonized the cecum in high numbers, but only enterococcus significantly limited the translocation of Escherichia coli C25 to mesenteric lymph nodes. However, the overall translocation rates were similar in all groups and ranged from 60% to 100%, due to translocation of other intestinal flora in addition to Escherichia coli C25. Conventionally reared mice were given either streptomycin, bacitracin/streptomycin or metronidazole which selectively eliminated facultative gram-negative bacteria, nearly all bacterial species or strictly anaerobic bacteria respectively. Only metronidazole significantly increased the rates of translocation of normal intestinal bacteria into mesenteric lymph nodes. Cohort groups of mice were then orally inoculated with drug resistant Escherichia coli C25, which actively colonized the cecum of all drug treated mice and translocated to the mesenteric lymph nodes of approximately half the streptomycin and metronidazole treated mice and nearly all the bacitracin/streptomycin treated mice. These results indicate that anaerobic bacteria play a pivotal role in limiting the translocation of normal intestinal bacteria, but that other bacterial groups also have a role in preventing the intestinal colonization and translocation of potential pathogens.

Role of anaerobic flora in the translocation of aerobic and facultatively anaerobic intestinal bacteria

It is thought that the normal enteric microflora acts not only to prevent intestinal colonization but also to prevent subsequent systemic dissemination of ingested, potentially pathogenic bacteria. To determine the relative roles of specific components of the intestinal bacterial flora in bacterial translocation out of the gut, mice were given various antimicrobial agents to selectively eliminate specific groups of intestinal bacteria. The cecal flora and the translocating bacteria in mesenteric lymph nodes were monitored both before and after oral inoculation with antibiotic-resistant Escherichia coli C25. Orally administered streptomycin selectively eliminated cecal facultative gram-negative bacilli, orally administered bacitracin-streptomycin eliminated all cecal bacterial species except low numbers of aerobic sporeformers, and parenterally administered metronidazole selectively eliminated cecal anaerobic bacteria. Compared with control mice, only metronidazole-treated mice had significantly increased rates of dissemination of intestinal bacteria into mesenteric lymph nodes, indicating that the exclusive absence of anaerobic bacteria facilitated the translocation of the intestinal facultative bacteria. In a parallel experiment with streptomycin-resistant E. coli C25 as a marker, parallel results were obtained. Metronidazole increased the translocation of the marker strain and the indigenous strains of intestinal bacteria. Thus, anaerobes appeared to play a key role in confining indigenous bacteria to the gut. However, intestinal colonization and translocation of E. coli C25 occurred most readily after bacitracin-streptomycin treatment, suggesting that in addition to anaerobic bacteria, other bacterial groups may play a role in limiting the intestinal colonization and extraintestinal dissemination of E. coli C25.