In this section four unrelated, but very important human pathogens are considered.
Streptococcus pneumoniae (“pneumococcus”), is a major cause of morbidity and mortality in humans. The organism is the most frequently identified causative agent of acute bacterial pneumonia, a common reason for admission to hospital. Streptococcus pneumoniae is a gram-positive coccus that, like “viridans” streptococci, are alpha hemolytic.
Its outstanding characteristic is an ample polysaccharide capsule that shields it from phagocytosis and is highly antigenic. The capsule is a principal virulence factor of the organism and elicits a powerful inflammatory reaction.
Microbiologic examination of sputum is fraught with error, as any specimen of lung secretion must pass through the heavily colonized oropharynx. The laboratory applies microscopic criteria to assess the suitability of sputum for culture by determining the presence of buccal epithelial cells (indicating oral contamination) or polymorphonuclear leukocytes (indicating an active inflammatory response in the lung and specimen from the lower tract). If the laboratory determines that the sample is of oral origin it will be rejected. See Lab – Sputum and tracheal aspirate
However, if the patient has the ability to produce a sample from the lower respiratory tract, examination of sputum by Gram stain may be a useful, rapid method for diagnosing pneumococcal pneumonia. The organism will appear as lancet-shaped diplococci surrounded by an unstained capsule. S. pneumoniae sputum Gram stain
On blood agar, pneumococcal colonies are surrounded by an area of alpha hemolysis. As viridans streptococci, which inhabit the oropharynx, are also alpha-hemolytic, S. pneumoniae must be distinguished by special tests.
Pneumococcal pneumonia has been referred to as the “Captain of the Men of Death” and even with a wide array of effective antibiotics available, pneumonia due to this organism remains a common cause of death. Most disease is sporadic though epidemics in closed quarters have been well described.
Classically, disease begins with a rigor – a violent, uncontrollable shaking followed by high fever. Cough develops and becomes productive of rusty colored sputum. Pleuritic chest pain is common. Bacteremia may accompany pneumonia and is a bad prognostic sign.
CXR usually reveals a dense consolidation in a “lobar” pattern.
Since the widespread use of Haemophilus influenzae type B vaccine has nearly eliminated disease caused by this bacterium, S. pneumoniae has become the most common cause of childhood bacterial meningitis. It also causes disease in adults and must be “covered” for whenever antibiotic therapy for this most serious of infections is started. This is becoming a difficult task owing to rapidly increasing resistance levels of S. pneumoniae to several classes of antibiotics.
Otitis Media and Sinusitis
In studies, S. pneumoniae is frequently isolated from patients with both of these upper respiratory tract infections. However, because sampling of the middle ear and sinuses is very difficult, empiric therapy for patients is the norm. Caution must be exercised, however as over-prescription of antibiotics for viral upper respiratory tract infections is a large contributing factor in the development of antibiotic resistance making therapy of true infections with S. pneumoniae increasingly difficult.
S. pneumoniae vaccine (Pneumovax) has been available for many years and is advocated for patients with any chronic illness and all persons over age 65. It has never achieved widespread use, in part because it is not effective in children under 2 years of age.
However, new protein conjugate vaccines (e.g. Prevnar) similar in concept to the very successful Haemophilus influenzae Type B vaccine (see below) are now included in universal childhood immunization programs. Early evaluations of effect are very promising and important in light of the increase in antibiotic resistance limiting therapy options.
Haemophilus spp. are small Gram negative bacilli that are part of the normal flora found on the mucous membranes of humans and other animals. There are several species including Haemophilus parainfluenzae, H. hemolyticus and H. parahemolyticus that almost never cause disease. Haemophilus influenzae is, by far, the most important pathogen. Haemophilus ducreyi Is the etiologic agent of chancroid – an ulcerative STD discussed in another section.
The word Haemophilus is derived from the Latin for “blood loving” and organisms belonging to this genus require blood for their propagation in vitro. In particular, H. influenzae requires hemin (X factor) and nicotinamide adenine dinucleotide or NAD (V Factor) for growth and these must be supplied in the media. A commonly used technique for supplying these two growth factors is the application of a “Staphylococcus streak” to a blood agar plate used for primary innoculation of respiratory samples. The hemolytic S. aureus liberates hemin from the blood and produces NAD in excess of its own requirements. Hemophilus spp. only grow in close proximity to the streak, a phenomenon referred to as “satellitism”. Haemophilus satellitism
Haemophilus influenzae acquired its name because it was mistakenly thought to be the primary cause of influenza during a time when bacteriology was in its infancy and viruses not discovered. It is a general respiratory tract pathogen and, while much less common than S. pneumoniae, is a well described cause of acute community-acquired pneumonia. It is also on the list of potential bacterial causes of otitis media and sinusitis.
Strains of Haemophilus influenzae are broadly divided, based upon the presence or absence of a polysaccharide capsule, into “typable” – those that possess a capsule, and “non-typable” those that do not possess a capsule. The only capsular serotype of note is B and H. influenzae type B (Hib), was an extremely important pediatric pathogen until the advent of a very successful vaccine.
Diseases caused by Hib are very different than those associated with non-typable strains.
Clinical Disease Caused by H. influenzae type B (Hib)
Until the advent of universal immunization, H. influenzae type B was the most common cause of severe invasive bacterial infections in children. Fortunately meningitis, epiglotitis, bacteremia, and septic arthritis caused by Hib are now very rare.
Clinical Disease Caused by non- typable H. influenzae
The large majority of disease now associated with H. influenzae is in the respiratory tract. Non-typable strains are common causes of otitis media, sinusitis in children and adults and the most common bacterial cause of exacerbations of chronic bronchitis in patients with chronic obstructive pulmonary disease (COPD), most often caused by cigarette smoking.
Bordetella are extremely small, strictly aerobic gram-negative bacilli. The principle species in this genus is Bordetella pertussis the agent responsible for pertussis or whooping cough.
Bordetella pertussis is identified by its characteristic microscopic and colonial morphology on selective media and its reactivity with specific antisera. The laboratory must be alerted to the possibility of B. pertussis being sought, as it will only grow on very selective media and is not routinely looked for.
Infection with Bordetella pertussis and the development of whooping cough require exposure to the organism, bacterial attachment to the ciliated epithelium cells of the bronchial tree, proliferation of the bacteria and the production of localized and systemic tissue damage. Filamentous hemagglutinin and pertussis toxin are responsible for bacterial attachment to ciliated epithelial cells. The toxins act locally to damage ciliated epithelial cells, which subsequently are unable to clear mucus from the respiratory tract.
There are three phases in this infection – the catarrhal stage resembling a common cold with serous rhinorrhea, sneezing and a mild cough. After one to two weeks the paroxysmal stage begins with classic whooping cough paroxysms. The paroxysms are characterized by a series of repetitive coughs followed by an inspiratory whoop. After several more weeks the disease enters the convalescent stage and the paroxysms diminish in number and severity.
Routine childhood immunization is relatively effective in reducing the incidence of disease however the use of prophylactic antibiotic treatment of contacts is also occasionally employed during outbreaks.
Neisseria meningitidis (“meningococcus”) is an encapsulated gram-negative diplococcus. Meningococci form small, translucent colonies on chocolate agar and are differentiated from other Neisseria spp. by carbohydrate utilization and serological tests. Neisseria meningitidis is subdivided into serogroups with types A, B, C, Y and W135 most commonly associated with meningococcal disease.
The major diseases caused by this organism are meningitis and septicemia (meningococcemia). Meningococci cause devastating disease in people of all ages with incidence being highest in children and young adults. It is usually fatal without prompt therapy with intravenous antibiotics and is one of the few true infectious emergencies. Hallmarks of severe disease are petechial skin lesions that can coalesce to form larger hemorrhagic lesions. Meningococcal rash
Fortunately, disease is sporadic and rare with an incidence of approximately 1:100,000.
A quadra-valent polysaccharide vaccine has been available for many years. It is not universally administered as it is a polysaccharide vaccine and not effective in small children. It is sometimes employed in the control of outbreaks, in military recruits and in travelers to hyper endemic areas (in particular sub- Saharan Africa). A conjugate type C vaccine has recently become a part of routine immunization schedules in Canada. A Tetravalent vaccine (Menactra) is likely to replace the C-only vaccine in the near future.
Post-exposure prophylactic antibiotic therapy is used to reduce the incidence of secondary cases in persons that have been in very close contact with a case.