Bacterial Infections

Noncholera gastroenteritis is often associated with recent ingestion of raw or undercooked shellfish and often occurs in the summer or early fall in the United States. A typical presentation includes explosive diarrhea, low-grade fever and chills, and mild-to-moderate abdominal pain with cramping.

Cholera is a fulminant diarrheal disease caused by V cholerae (Box 1). Patients with cholera develop frequent, watery stools, which may reach volumes =1 L/h.

Before 1990, strains of Haemophilus influenzae type b were found in the upper respiratory tract of 3-5% of children and a small percentage of adults. Colonization rates with type-b strains are even lower now, reflecting routine immunization of infants against H influenzae type b. Non-type-b encapsulated H influenzae are present in the nasopharynx of < 2% of individuals, whereas nonencapsulated (nontypable [see below]) strains colonize the respiratory tract of 40-80% of children and adults.

B catarrhalis causes bronchitis and pneumonia in patients with underlying lung disease, especially chronic obstructive pulmonary disease. It is also a rare cause of invasive disease, including meningitis, endocarditis, bacteremia without a focus, septic arthritis, and cellulitis.

The catarrhal stage is characterized by nonspecific upper respiratory symptoms, including rhinorrhea, mild cough, and low-grade fever. During this stage, which typically lasts 1-2 weeks, the disease is highly communicable. The paroxysmal stage is marked by sudden attacks or paroxysms of severe, repetitive coughing, often culminating with the characteristic whoop and frequently followed by vomiting.

H influenzae was first isolated during the 1892 influenza pandemic and was originally believed to be the causative agent of influenza. Although subsequent studies revealed the fallacy of this idea, H influenzae has proved to be a common cause of localized respiratory tract and systemic disease, including meningitis, epiglottitis, pneumonia, pyogenic arthritis, cellulitis, otitis media, and sinusitis, among others (Box 1). Meningitis is the most common and serious form of invasive H influenzae type-b disease. In the mid-1980s, before the introduction of effective vaccines, ~ 10,000-12,000 cases of H influenzae type-b meningitis occurred in the United States each year, and 95% of cases involved children < 5 years old.

Pathologists have noted spiral bacteria in biopsies and autopsy specimens of gastric mucosa for over 100 years. Their significance was alternately debated and ignored until 1982, when Barry Marshall and Robin Warren cultivated the organism for the first time and suggested that it might be a cause of chronic gastritis and peptic ulcer disease. Although initially called Campylobacter pylori, subsequent taxonomic studies showed that the bacterium was not a true Campylobacter species, and it was renamed Helicobacter pylori.

The genus Pseudomonas consists of a number of human pathogens, the most important of which is Pseudomonas aeruginosa. P aeruginosa is an opportunistic pathogen found widely in soil, water, and organic material, reflecting its limited nutritional requirements. A moist environment is favored. Human colonization in the community is rare, and, when it occurs, the skin, gut, and upper or lower airway are colonized.

This organism is endemic in Southeast Asia with the highest prevalence in Thailand. The organism is a saprophyte living in the soil. Infection may be subclinical, acute, subacute, or chronic.

Paeruginosa infections may occur in patients with AIDS. Risk factors for infection include a CD4 count of < 100 cells/mL3, neutropenia or functional neutrophil defects, intravascular catheterization, hospitalization, and prior use of antibiotics including ciprofloxacin or trimethoprim-sulfamethoxazole. Many cases are community acquired. Bacteremia is common, and the lung or an intravenous catheter is the most frequent portal of entry.