Summary. As few as five of the species of bacteria commonly found in human faecesEscherichia coli, Enterobacter aerogenes, Enterococcus faecalis, Bacteroides ovatus and Fusobacterium variurn-when grown together in anaerobic continuous flow cultures exerted antagonistic effects on Salmonella typhimurium as great as those given by mixed bacteria from extracts of human faeces. In a single culture, the population of S. typhimurium was c. 10' cfu/ml but in mixed cultures with the five antagonistic bacteria or mixed faecal bacteria it was reduced to c. lo3 cfu/ml. Antagonism appeared to be the result of competition for the growth limiting amino acids, arginine, serine, threonine and aspartic acid. Optimal manifestation of antagonism required the presence of carbon sources fermentable only by antagonistic bacteria, such as lactose 0-1 %, w/v, sucrose 0-1 % (w/v) and starch 0.2-0-3% w/v. These carbohydrates promoted the growth of the antagonistic bacteria, particularly E. coli and B. ovatus. However, an increase in concentration by several fold of any one of four growth-limiting amino acids in the medium diminished the antagonistic effects and the population of S. typhimurium rose 102-103-fold.
Summary. The growth of nine species of colonic bacteria-Escherichia coli, Enterococcus faecalis, Bacteroides ovatus, Fusobacteriurn varium, Clostridiurn perfringens, Klebsiella pneurnoniae, Proteus vulgaris, Staphylococcus aureus and Bifidobacteriurn adolescentis-was examined after concomitant injection to form experimental subcutaneous abscesses in mice. Injection of a mixture of c. lo5 cfu of each of the first five strains (E. coli, Ent. faecalis, B. ovatis, F. variurn and C. perfringens) resulted in abscess formation in all mice tested when the E. coli strain was haemolytic. E. coli and B. ovatus multiplied and reached a maximum population of c. lo8 cfu/abscess. When non-haemolytic E. coli was used, injection of 2 lo7 cfu was required for abscess formation. The inclusion of partially purified E. coli haemolysin (1 25 HUsO) with c. 10' cfu of bacteria including non-haemolytic E. coli resulted in abscess formation in most mice tested. These results indicate that E. coli haemolysin is one factor that may potentiate pathogenic synergy among colonic bacteria especially between E. coli and B. ovatus, during abscess formation.
IntroductionA wide variety of aerobic and anaerobic bacteria has been isolated from infected foci, such as abscesses, caused by colonic flora. Escherichia coli and members of the fragilis group of Bacteroides are detected most frequently. ,* Since these bacteria usually predominate in the colonic flora, their frequent isolation is to be expected. However, the reasons for the selective survival of E. coli and B. fragilis in such foci of infection are not clear. Many investigations have reported pathogenic synergy among aerobic and anaerobic bacteria. 3-' Verweijvan Vught et a1.6 found synergy between E. coli and B. fragilis in abscess formation, and showed that the intensity of synergic activity was dependent on E. coli although the factors involved were not identified. In this study, the role of haemolysin produced by E. coli in the pathogenic synergy of nine bacterial species predominant in the colonic microflora was examined in a subcutaneous abscess model in mice.
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