The bacteria that cause infection are most commonly part of the indigenous bacteria that normally live on or in the host. Odontogenic infections are no exception, because the bacteria that cause odontogenic infections are part of the normal oral flora: those that comprise the bacteria of plaque, those found on the mucosal surfaces, and those ,found in the gingival sulcus. They are primarily aerobic gram-positive cocci, anaerobic gram-positive cocci, and anaerobic gram-negative rods. These bacteria cause a variety of common diseases, such as dental caries, gingivitis, and periodontitis. When these bacteria gain access to deeper underlying tissues, as through a necrotic dental pulp or through a deep periodontal pocket, they cause
odontogenic infections. Many carefully performed microbiologiC studies of
odontogenic infections have demonstrated the microbiologic composition of these infections. Several important factors must be noted. First, almost all odontogenic infections. are caused by multiple bacteria. The polymicrobial
nature of these infections makes it important that the clinician understand the variety of bacteria that are likely to

TABLE 15-1

Causative Organisms

Causative Organisms”
Anaerobic only’
Num~er I:!f Patients

In 404 patients; data from Aderhold L, Konthe H, Frenkel GThe baCteriology of dentogenous pyogenic infections, OralSu:g 52:583,
1981Bartlett JG,O’Keefe P: The bacteriology of perimandibular space infections, J 6ral Surg 50: 130, 1980; ‘Chow Aw, Roser 5M, Brady FA: Orofacial odontogenic infections, Ann Intern Med 88:392, 978; lewis MAO et al: Prevalence of penicillin resistant bacteria in ~ e suppurative oral infection, J Antimicrob Chemothe; 35B:785, 5- McCowan DA: Is antibiotic prophylaxis required far dental’ oat~~.”.o’Ith joint replacement? Br Dent J 158:336, 1985; Norden ~,,: PrNentiof’l of bone andjomt infections, Am J Med78:229, i985. cause the infection. In most odontogenic intectlons the
laboratory can identify an average of five species of bacteria. It js Dot unusual for as many as eight different species to be identified in a given infection. On rare occasions a single species may be isolated. A second important factor is the anaerobic-aerobic characteristic of the bacteria causing odontogenic infec- . < tions. Because the mouth flora is ,a combination of aerobic and anaerobic bacteria, it is not surprising to find that most odontogenic infections have both anaerobic and aerobic bacteria. Infections caused by only aerobic. bacteria probably account for 5% of all odontogenic infections. Infections caused by only anaerobic bacteria make
up about 35% of the infections. Infections caused by both anaerobic and aerobic bacteria comprise about 60% of all odontogenic infections (Table 15-1).   The aerobic bacteria that cause odontogenic infections consist of many species (Table 15-2).<The most common causative organisms are streptococci, which comprise about 90% of the aerobic bacterial species that cause odontogenic infections. Staphylococci account for about
5% of the aerobic bacteria, and many miscellaneous bacteria contribute 10/0 or less. Rarely found bacteria include group D Streptococcus organisms, Neisseria spp., Corynebac

TABLE 15-2

Microorganisms Causing Qdontogenic Infections”

Organism, Percentage
Aerobict 25 .
Gram-positive cocci 85
Streptococcus spp: 90
Streptococcus (group D) spp. 2
Staphylococcus spp. 6
Eikenella spp. ‘:2
Gram-negative cocci (Neisseria spp.) 2
Gram-positive rods (Corynebacterium spp.) 3
Gram-negative rods (Haemophilus spp.) . 6
Miscellaneous and undifferentiateCl 4
Ancierobict 75
Gram-positive cocci 30
Streptococcus spp. 33
Peptostreptococcus spp. 65
Gram-negative cocci (Veillonella spp.) 4
Gram-positive rods 14
Eubacterium spp.
Lactobacillus spp.
Actinomyces spp.
Clostridia spp.
Gram-negative rods 50
Bacteroides 75
Fusobacterium spp. 25
Miscellaneous 6

The anaerobic bacteria that cause infections include an even greater variety of species (see Table 15~2). Two main groups, however, predominate. The anaerobic grampositive cocci ‘account for about one third of the bacteria.
These cocci are anaerobic Streptococcus and Peptostreptococcus. The’ gram-positive rods Eubacterium and LactbbacilIus organisms are most commonly found in this group . . The gram-negative anaerobic rods are cultured in about  alf of the infections. The Prevotella and Porphyromonas
(previously Bacteroides) spp. account for a out 75% ofthese, and  usobacterium organisms account for 25%. Of the anaerobic bacteria, several gram-positive cocd (i.e., anaerobic Streptococcus and Peptostreptococcus spp.) and  ram-negative coos (i.e., Prevot6lla and Fusobacterium spp.) play a more important pathogenic roe. The anaerobic
gram-negative cocd and the anaerobic gram-positive rods appear to have little or no role in ‘the cause of odontogenic infections; instead they appear to be opportunistic organisms. The method by which mixed aerobic and anaerobic  bacteria cause infections i~ known with some certainty. . After init al inoculation into the deeper tissues, the more :-waslve organisms with higher virulence (i.e., the aerobic -ireptococcus spp.) begin the infection process, initiating a cellulitis type of infection. The anaerobic bacteria will
then also grow, and   s the local reduction-oxidation potential is lowered (because of the growth of the aerobic bacteria),  naerobic bacteria become more prominent. As the infection reaches a more chronic, abscess stage, the anaerobic bacteria predominate and eventually become the exclusive causative organisms. Early infections appearing initially as a cellulitis may be characterized as  erobic streptococcal infections, and late, chronic abscesses may be characterized as anaerobic infections ..

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