Campylobacter, Helicobacter, Bacteroides, and Mycoplasma Flashcards
What are the morphologic features of Campylobacter?
- Small, motile, curved, Gram-negative rods
- Microaerophilic
- C. jejuni grows better at 42ºC than at 37ºC
- Express lipooligosaccharides (which lack the O-antigen of LPS)
- Killed when exposed to gastric acids
What are the features of disease caused by Campylobacter?
- The most common cause of bacterial gastroenteritis, with C. jejuni responsible for most infections
- C. jejuni produces histologic damage to the mucosal surfaces of the jejunum and other parts of the intestine
- Presents most commonly as acute enteritis with diarrhea, fever, and severe abdominal pain
- Guillain-Barré syndrome and reactive arthritis are uncommon complications, presumably through molecular mimicry
What is the epidemiology of Campylobacter?
- Zoonotic infections, with contaminated poultry as the main reservoir
- Uncommon for the disease to be transmitted by food handlers
How is Campylobacter diagnosed?
Identification of isolates based on growth under selective conditions, microscopic morphology, and positive oxidase and catalase tests
What are the morphologic features of Helicobacter?
- Spiral Gram-negative rods that resemble campylobacters
- Highly motile (via corkscrew motility)
- Produce an abundance of urease
- Require a complex medium in microaerophilic conditions
What is the epidemiology of Helicobacter?
- Humans are the primary reservoir of H. pylori
- Colonization persists for life unless the host is specifically treated
- Transmission is most likely via the fecal–oral route
What is the pathogenesis of H. pylori infection?
- H. pylori use their motility, chemotaxis, urease production, and other mechanisms to adapt to the acidic conditions of the stomach and colonize a narrow protected niche near the surface of epithelial cells
- Localized tissue damage is mediated by urease byproducts, mucinase, phospholipases, and the activity of vacuolating cytotoxin A (VacA), a protein that damages invaded cells by producing vacuoles. Cytotoxin-associated gene (cagA) interferes with the normal cytoskeletal structure of the epithelia
What are the features of H. pylori infection?
- Acute-phase gastritis: a feeling of fullness, nausea, vomiting, hypochlorhydria
- Chronic gastritis, with the disease confined to the gastric antrum or involving the entire stomach
- Chronic gastritis progresses to develop peptic ulcers, commonly involving the junction between the corpus and antrum (gastric ulcer) or the proximal duodenum (duodenal ulcer)
- Chronic gastritis increases the risk of gastric cancer and MALT lymphoma
How is Helicobacter diagnosed?
- Histologic examination of gastric biopsy specimens
- Polyclonal and monoclonal immunoassays for H. pylori antigens in the stool
- Urea breath test
What are the morphologic features of Vibrio?
- Gram-negative, facultatively anaerobic, fermentative rods
- Characterized by a positive oxidase reaction and the presence of polar flagella
- Can grow in a broad temperature range and a broad pH range but are susceptible to stomach acids
- All Vibrio spp. require NaCl for growth, most are halophilic
- Grow in estuarine and marine environments worldwide, as well as in chitinous shellfish
How is Vibrio cholerae subdivided into serogroups?
Based on the O polysaccharide of LPS
Which serogroups of V. cholerae produce the cholera toxin?
- O1
- O139
What is the epidemiology of Vibrio?
- Spread by contaminated water and food
- A high inoculum is required to establish infection
- Prominent in communities with poor sanitation
- 3–5 million cases occur every year
What is the pathogenesis of cholera?
- The cholera toxin is a complex A-B toxin. The active portion of the A subunit is internalized and interacts with G proteins that control adenylate cyclase, increasing levels of cAMP. This leads to hypersecretion of water and electrolytes
- Toxin coregulated pilus on vibrio pathogenicity island-1: a surface binding protein for the bacteriophage CTXΦ that encodes the genes for the subunits of the cholera toxin
What are the features of cholera?
- Clinical manifestations begin 2–3 days after ingestion with the abrupt onset of watery diarrhea and vomiting. Fever is rare
- The feces-streaked stool specimens become colorless and odorless, free of protein, and speckled with mucus (“rice-water” stools)
- Severe fluid and electrolyte loss can lead to dehydration, painful muscle cramps, metabolic acidosis (due to bicarbonate loss), hypokalemia, and hypovolemic shock with cardiac arrhythmia and renal failure
- Mortality rate is 70% in untreated patients but <1% in patients who receive treatment
How is cholera treated?
Prompt fluid and electrolyte replacement before hypovolemic shock can occur
What are the morphologic features of Mycoplasma?
- The smallest free-living bacteria
- They do not have a cell wall and their cell membrane contains sterols
- The mycoplasma form pleomorphic shapes, varying from coccoid forms to rods
- M. pneumoniae is a strict human pathogen
What are the diseases caused by mycoplasmas?
- M. pneumoniae: respiratory disease, tracheobronchitis, pneumonia (atypical) with a patchy bronchopneumonia seen on chest radiographs
- M. genitalium: nongonococcal urethritis, pelvic inflammatory disease
How are mycoplasmas diagnosed?
- PCR amplification of species-specific gene targets
- Antigen tests have poor sensitivity and specificity
- Microscopy is of no diagnostic value as mycoplasmas do not stain well with the Gram stain
What are the morphologic features of Bacteroides?
- Anaerobic Gram-negative pleomorphic cells
- Growth is stimulated by bile
- May be surrounded by a polysaccharide capsule
- Bacteroides LPS has little endotoxin activity due to differences in its structure
How is Bacteroides fragilis transmitted?
Endogenous bacteria spread by trauma or disease from the normally colonized mucosal surfaces of the ileum and colon to sterile tissues or fluids
What are the diseases caused by Bacteroides fragilis?
- Respiratory tract infections, chronic infections of the sinuses and ears, and virtually all periodontal infections (alongside other Gram-negative anaerobes)
- Strains of enterotoxigenic B. fragilis produce a heat-labile zinc metalloprotease toxin that produces a self-limited watery diarrhea
- Bacteremia
- Intraabdominal infections
- Skin and soft-tissue infections, usually in immunocompromised patients
