ABSTRACT
Anaerobic bacteria differ in their pathogenicity. Not all of them are believed to be clinically
significant, while others are known to be highly pathogenic. Table 1 lists the major anaerobes
that are most frequently encountered clinically. The taxonomy of anaerobic bacteria has
changed in recent years because of their improved characterization using genetic studies (1).
The ability to differentiate between similar strains enables better characterization of type of
infection and predicted antimicrobial susceptibility. The species of anaerobes most frequently
isolated from clinical infections are in decreasing frequency: the clinically important anaerobes
are of gram-negative rods (Bacteroides, Prevotella, Porphyromonas, Fusobacterium, Bilophila and
Sutterella), gram-positive cocci (primarily Peptostreptococcus), gram-positive spore-forming
(Clostridium) and non-spore-forming bacilli (Actinomyces, Propionibacterium, Eubacterium, Lacto-
bacillus, and Bifidobacterium), and gram-negative cocci (mainly Veillonella) (2). About 95% of the
anaerobes isolated from clinical infections are members of these genera. The remaining isolates
belong to species not yet described, but these usually can be assigned to the appropriate
genus on the basis of morphologic characteristics and fermentation products. The frequency
of recovery of the different anaerobic strains differs in various infectious sites. The 12 years
experience in recovering anaerobic bacteria from adults and children at two medical centers is
presented in Table 2 (3). The main isolates were anaerobic gram-negative bacilli (Bacteroids,
Prevotella, and Porphyromonas; 43% of anaerobic isolates), anaerobic gram-positive cocci (26%),
Clostridium spp. (7%), and Fusobacterium spp. (5%). This chapter discusses the main anaerobic
species and their role in infectious processes.
