1. By far the most frequent cause of bacterial pharyngitis and tonsillitis is Streptococcus pyogenes (Lancefield group A). This infection is particularly prevalent in young children (5-12 years). When streptococcal pharyngitis is associated with a characteristic skin rash, the patient is said to have scarlet fever. In infants, a streptococcal throat infection may often involve the nasopharynx and be accompanied by a purulent nasal discharge.
Non-group-A, β-haemolytic streptococci (e.g., groups B, C and G) are uncommon causes of bacterial pharyngitis and if detected should be reported. Pharyngeal infections due to S. pyogenes, if not properly treated, may give rise to sequelae such as rheumatic fever, and less often glomerulonephritis. Specific identification of, and antibacterial treatment directed against, S. pyogenes are primarily intended to prevent the occurrence of rheumatic fever. Recurrent pharyngitis with S. pyogenes, in spite of correct treatment with penicillin, may be caused by β-lactamase-producing commensals in the pharynx (Staphylococcus, Haemophilus, Branhamella, and anaerobes).
2. Corynebacterium diphtheriae is the cause of diphtheria, a disease that is endemic in many tropical areas. Characteristically (with a few exceptions), C. diphtheriae causes a typical form of infection, characterized by a -white membrane at the site of infection (pharynx, tonsils, nose, or larynx). Diphtheria is a serious disease and the diagnosis is made on the basis of clinical findings. The physician would then generally make a specific request to culture for diphtheria bacilli. The microbiologist should be prepared to search for colonies that appear typical in “routine” throat cultures, especially in countries where the disease is prevalent.
3. Gonococcal pharyngitis has been recognized with increasing frequency in some countries, with rates that parallel the incidence of cervical and urethral gonorrhoea. Culture of throat swabs for gonococci should be done on specific request from the clinician, using the appropriate selective medium (modified Thayer-Martin medium).
4. Necrotizing ulcerative pharyngitis (Vincent angina) is a rare condition characterized by a necrotic ulceration of the pharynx with or without formation of a pseudomembrane. It is associated, at the site of infection, with a heavy mixed flora of strict anaerobes dominated by Gram-negative fusiform rods and spirochaetes, generally referred to as Fusobacterium spp and Borrelia vincentii, and possibly others. Although both species belong to the normal mouth flora, their presence in large numbers in a Gram-stained smear of ulcerated lesions should be reported as a “fusospirochaetal complex”. This microscopic diagnosis need not be confirmed by anaerobic culture, which is difficult and time-consuming. However, the presence of this complex does not exclude the need to search for other pathogens, particularly S. pyogenes.
5. Oral candidiasis. Although small numbers of C. albicans or other Candida species may be part of the normal oral flora, the number of organisms increases considerably in certain pathological conditions, e.g., in malnourished premature babies, in immunodeficient adults, or in patients who have received broad-spectrum antibiotics or cancer therapy. The affected area - tongue, tonsils, throat or buccal mucosa - may be intensively red, or covered by white patches or a confluent grey-white membrane (thrush). The diagnosis of candidiasis is best made by finding numerous yeast cells, some of them forming long mycelium-like filaments, in a Gram-stained smear of the exudate.
6. Search for healthy carriers. Swabs from the upper respiratory tract may be submitted to the laboratory, not for the diagnosis of a clinical infection, but to detect a potential pathogen in a healthy subject, a pharyngeal or a nasal “carrier”. This should only be done as part of well-defined epidemiological surveys. The following pathogens can give rise to a carrier state in the upper respiratory tract:
• Staphylococcus aureus. Sampling of patients and staff for nasal carriers is sometimes performed as part of an investigation of hospital outbreaks of S.
• Neisseria meningitidis. Carriage of meningococci may be very prevalent (20% or more) even at non-epidemic times. Identification of pharyngeal carriers of meningococci is rarely needed, and need not be performed prior to the administration of prophylactic antibiotics to family or other close contacts of patients with meningococcal disease.
• Streptococcus pyogenes. Carriage of this organism in low numbers is highly prevalent, especially among schoolchildren (20 - 30%).
• Corynebacterium diphtheriae. The carrier rate of the diphtheria bacillus is high in unimmunized populations. In such communities, it may be justified to identify and treat carriers among the close contacts of a patient with proven diphtheria. Carriers are rare when an immunization programme is correctly implemented.