14: Neisseriaceae, Veillonella, parvobacteria and Capnocytophaga

Chapter 14 Neisseriaceae, Veillonella, parvobacteria and Capnocytophaga

Neisseriaceae

The Neisseriaceae include the genera Neisseria and Moraxella. Two species of Neisseria are human pathogens:

• Neisseria gonorrhoeae (the gonococcus)
• Neisseria meningitidis (the meningococcus).

There are a number of non-pathogenic species, such as Neisseria sicca, Neisseria mucosa and Neisseria lactamica, which are members of the indigenous flora, including the oral mucosa. Hence, it is important to differentiate these from the pathogenic species from oral samples.

N. gonorrhoeae is the agent of gonorrhoea, the most frequently diagnosed venereal disease in western Europe and the USA. Gonococci frequently cause pelvic inflammatory disease (PID) and sterility in women, in addition to arthritis and sometimes septicaemia. N. meningitidis is the aetiological agent of meningococcal meningitis, a highly contagious disease associated with a mortality rate approximating 80% when untreated.

General characteristics

Non-motile, Gram-negative cocci ranging from 0.6 to 1.0 µm in diameter. On microscopy, the cocci are seen as pairs with concave adjacent sides (bean-shaped); tetrads, short chains and clusters are occasionally seen but all show the characteristic pairing.

Pathogenic Neisseria species are nutritionally fastidious, especially on initial isolation from clinical specimens; the non-pathogenic species are less so. Though aerobic, most strains of N. gonorrhoeae are capnophilic (they require increased carbon dioxide for growth); haemolysed blood and solubilized starch enhance growth.

Members of this genus grow optimally at 36–39°C, although the non-pathogenic species can grow at temperatures below 24°C.

Neisseria gonorrhoeae

Habitat and transmission

The human urogenital tract is the usual habitat; oral, nasopharyngeal and rectal carriage in healthy individuals is not uncommon. Spread is by both homosexual and heterosexual intercourse or intimate contact.

Characteristics

Non-motile, Gram-negative, non-capsulate diplococci.

Culture and identification

Specimens are usually inoculated onto an enriched medium (lysed blood or chocolate agar normally) and incubated under 5–10% carbon dioxide (as the species is capnophilic). Small, grey, oxidase-positive colonies initially become large and opaque on prolonged incubation. Subsequent staining by fluorescent antibody techniques, and the production of acid from glucose but not from maltose or sucrose, confirms the identification. Gram-stained smears (of urethral exudate from men and the cervix in women) usually reveal Gram-negative, kidney-shaped intracellular cocci in pairs.

Pathogenicity

Gonococci possess a number of virulent attributes:

• pili allow gonococci to adhere and colonize epithelial surfaces and thus cause infection
• immunoglobulin A (IgA) proteases produced by some gonococci break the heavy chain of IgA, thereby inactivating it (IgA is a major defence factor universally present on mucosal surfaces)
• some isolates of N. gonorrhoeae produce β-lactamase, which is plasmid-mediated
• a tracheal cytotoxin damages the ciliated cells of the fallopian tube, leading to scarring and sterility.

Treatment and prevention

The majority of gonococci are resistant to β-lactam drugs and hence the choice is β-lactamase-stable cephalosporins. Prevention of gonorrhoea requires the practice of ‘safe sex’, health education and contact tracing.

Neisseria meningitidis

Habitat and transmission

The main reservoir is the nasopharynx in healthy individuals (10–25%). Droplet spread is the most common transmission mode.

Characteristics

This organism resembles the gonococcus but N. meningitidis cells are capsulate.

Culture and identification

As for N. gonorrhoeae. Presumptive identification is made by observing Gram-negative cocci in pairs in nasopharyngeal discharge, cerebrospinal fluid or blood smears. Selective media are not required as the organism is found pure in cerebrospinal fluid. Identified by the carbohydrate utilization test: produces acid from the oxidation of glucose and maltose. Serology is useful.

Pathogenicity

In susceptible individuals, meningococci spread from the nasopharynx into the blood stream (septicaemia), and then to the meninges. Septicaemia is accompanied by a rash. Eventual death may be due to meningitis or adrenal haemorrhage (Waterhouse–Friderichsen syndrome). The antiphagocytic properties of the capsule help dissemination, while the toxic effects are mainly due to the meningococcal endotoxin.

Treatment and prevention

Penicillin or cefotaxime (or equivalent cephalosporin).

Commensal Neisseria species

Commensal Neisseria species are common in the oral cavity, nose and pharynx, and sometimes in the female genital tract. The taxonomy of the group is confused. The three main species are Neisseria subflava, N. mucosa and N. sicca. The main difference between these and the pathogenic Neisseria species is the ability of the commensal species to grow on ordinary agar at room temperature in the absence of carbon dioxide supplements.

These organisms are essentially non-pathogenic and are almost always found in oral specimens contaminated with saliva or mucosa. Neisseria species are among the earliest colonizers of a clean tooth surface. They consume oxygen during the early plaque formation and facilitate subsequent growth of facultative and obligate anaerobic late colonizers.

Moraxella

Moraxella (formerly Branhamella) are Gram-negative cocci closely related to the non-pathogenic Neisseria species, but asaccharolytic and non-pigmented. They are commensals of the human respiratory tract and are recognized opportunistic pathogens causing meningitis, endocarditis, otitis media, maxillary sinusitis and chronic obstructive pulmonary disease. As the majority of strains produce β-lactamase, they may indirectly ‘protect’ other pathogens and thus complicate antibiotic therapy.

Veillonella

Veillonella species are obligate anaerobic, Gram-negative cocci frequently isolated from oral samples. Three oral species are recognized: Veillonella parvula (the type species), Veillonella dispar and Veillonella atypica.

Veillonella parvula

Gram-negative, small anaerobic cocci. Found in the human oral cavity, mostly in dental plaque, they are considered as ‘benevolent organisms’ in relation to dental caries as they metabolize the lactic acid produced by cariogenic bacteria into weaker acids (acetic and propionic) with a reduced ability to solubilize enamel. No known pathogenic potential.

Parvobacteria

Parvobacteria are so called because of their size (Latin parvus: small). They are a miscellaneous, heterogeneous group of small, Gram-negative bacilli that cause a number of different diseases. They include the following genera:

• Haemophilus
• Brucella
• Bordetella
• Pasteurella (includes Aggregatibacter species)
• Francisella
• Gardnerella
• Eikenella.

Of these, Haemophilus and Bordetella spp. are of particular interest, as the former causes significant morbidity in the general population and the latter is the agent of whooping cough. Additionally, Haemophilus spp. and Aggregatibacter spp. are common inhabitants of the oral cavity; the latter being an important periodontopathogen.

Haemophilus spp.

The genus Haemophilus is composed of tiny, non-motile, aerobic, Gram-negative coccobacilli; some are capsulated. One of its major distinguishing features is the requirement of two growth factors:

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Related posts:

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  3. 11: Streptococci, staphylococci and micrococci
  4. 32: Microbiology of dental caries
  5. 26: Infections of the gastrointestinal tract
  6. 4: Viruses and prions

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Jan 4, 2015 | Posted by in General Dentistry | Comments Off on 14: Neisseriaceae, Veillonella, parvobacteria and Capnocytophaga

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