Actinomycetes are Gram-positive, catalase-positive, nonmotile bacilli. They are considered to be transitional forms between bacteria and fungi. Like bacteria they possess cell wall, contain-ing muramic acid. They also possess prokaryotic nuclei and are susceptible to antibiotics. But like fungi, they form delicate filaments called hyphae similar to the hyphal form in fungi. These hyphal forms are seen in bacteria isolated by culture and are also seen in clinical specimens.
The actinomycetes include a wide range of bacteria, which are found in soil and putrefied vegetables. They are also found in humans and animals. The actinomycetes include anaero-bic or facultative anaerobic bacteria belonging to the genus Actinomyces. Furthermore, depending on the presence orabsence of mycolic acids in the cell wall, aerobic actinomycetes can be broadly classified into two groups as follows:
· Actinomycetes with mycolic acids: This group includes-members of three families: Corynebacteriaceae, Mycobac-teriaceae, and Nocardiaceae. The family Nocardiaceae consists of four genera: Nocardia, Rhodococcus, Tsukamurella, and Gordonia. Members of all these genera stain poorly with Gram stain and are partially acid fast.
· Actinomycetes without mycolic acid: This groupincludes many opportunistic pathogens, such as Actino-madura, Nocardiopsis, Streptomyces, Dermatophilus, Oerskovia,Rothia, Tropheryma, and thermophilic actinomycetes Saccharopolyspora, Saccharomonospora, and Thermoactinomyces.
Human infections caused by actinomycetes are summarized in Table 46-1.
Actinomyces israeli is the most common Actinomyces causinghuman infection. Other species are Actinomyces gerencsonei, Actinomyces turicensis, Actinomyces radingae, Actinomyces europaeus, Actinomyces naeslundii, Actinomyces odontolyticus, Actinomyces viscosus, Actinomyces meyeri, and Propionibacterium propionicum.