Gram-positive bacteria are common commensal bacteria of the skin and mucosal surfaces that can cause disease with clearly defined clinical syndromes. Streptococci are usually penicillin sensitive, while staphylococci are often penicillin resistant. The epidemiology of streptococcal and staphylococcal disease, and in particular patterns of antibiotic sensitivity are not well described across Africa (Lubell et al., 2010; Reddy et al., 2010).
The streptococci are a group of 30 species of bacteria. These are usefully divided according to the appearance of their colonies on blood agar plates. Colonies with a surrounding green pigmented zone (described as alpha-haemolysis) are Streptococcus pneumoniae or one of the large groups of Streptococcus viridans, which are common oral and nasopharyngeal flora only occasionally causing severe disease. Non-haemolytic streptococci are rarely pathogenic. Streptococcal colonies with a surrounding zone of complete haemolysis (clear appearance) are called beta-haemolytic streptococci. These are serologically classified into groups A to H and K to V. Of these, Group A beta-haemolytic streptococci (also known as Streptococcus pyogenes) are by far the most important in causing human disease, and form the subject of this chapter. Some Group B streptococci may cause puerperal sepsis and neonatal infections (including meningitis) in Africa, and Groups C, D and G organisms may cause upper respiratory infections, urinary infections and endocarditis.
The problem in Africa
For many decades pneumonia has ranked second only to malaria as a cause of admission to adult medical wards across Africa (Harries et al., 1990; Williams et al., 1986). However, in the last 25 years the pattern of hospital admissions has changed, reflecting the dominance of HIV-related problems. The two commonest reasons for admission among HIV-infected adults are tuberculosis and pneumonia (Grant et al., 1998), so pneumonia continues to present a considerable burden to hospital services across the continent.
Pneumonia affects economically productive people. Although its incidence is higher among those with chronic underlying diseases, the typical patient in Africa is a healthy individual with a short history of illness. Although mortality is higher in the elderly, the majority of pneumonia-related deaths in hospital occur among young adults, less than 40 years old. Furthermore, among survivors, 60 per cent of pneumonia patients have not recovered sufficiently to return to work 3 weeks after their admission (Scott et al., 2000).
Pneumonia is usually straightforward to diagnose and treat. The challenges in management are identifying the severely ill patient, predicting the aetiology of disease, dealing with resistance among respiratory pathogens and anticipating complications.
Streptococcus pneumoniae is a major cause of morbidity and mortality worldwide, and commonly colonizes the upper respiratory tract. In some colonized individuals the organism translocates to other tissues and causes life-threatening diseases including pneumonia, bacteremia, and meningitis. Rates of disease are especially high in the very young and old and in patients with predisposing conditions including HIV infection, cardiopulmonary or co-morbidities, renal diseases including nephrotic syndrome, and sickle cell disease and other causes of hyposplenia.
Approximately 5 million children under the age of 5 years die with a respiratory tract infection every year; the major causative pathogen in these cases is S. pneumoniae (Williams et al. 2002). Pneumococcal infection caused approximately 45,000 deaths in adults in the United States in 1998, about two years prior to the introduction of the 7-valent pneumococcal conjugate vaccine for infants (Robinson et al. 2001). Streptococcus pneumoniae also causes less serious but extremely common diseases such as otitis media, sinusitis and exacerbations of chronic obstructive pulmonary disease. It has been estimated that there are 7 million cases of otitis media in the United States every year (Stool & Field 1989).
OVERVIEW OF PATHOGENESIS
The first step in the pathogenesis of pneumococcal disease is nasopharyngeal colonization, with individuals often carrying more than one serotype at a time. Asymptomatic nasopharyngeal carriage is established when surface components of the pneumococcus bind to nasal epithelium (Tuomanen & Masure 1997; Weiser et al., 1996).
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