Studies of the cultivable flora of normal human feces

Highly diluted feces, obtained from healthy adult individuals, was plated on blood-agar plates which were incubated both aerobically and anaerobically. From the anaerobic plates containing 30 to 60 colonies, every colony was subcultured. Nearly all isolates were obtained in pure culture and partially characterized. It was found thatBacteroides species were the most predominant organisms, being present in numbers approximating 1010 per gram wet weight. Selected bacteria present in lower numbers were determined by plating appropriate dilutions of feces on selective media. It was found that coliforms, streptococci and lactobacilli were regularly present in concentrations of 106 − 108 organisms per gram wet weight material, whileVeillonella, Streptococcus salivarius, Bacteroides melaninogenicus and staphylococci were present in lower numbers. Fusobacteria were only found in one sample, whileNeisseria were not detected in any of the samples. Wet mounts of fecal material, inspected by darkfield microscopy, did not reveal the presence of spirochetes. Anaerobes outnumbered facultative bacteria by a factor of 40, indicating that the human adult fecal flora is predominantly anaerobic. Total microscopic counts indicate that bacteria comprise approximately 30% of the mass of human feces.