Non–spore-forming Gram-positive bacteria Flashcards
What are the non–spore-forming Gram-positive bacteria of clinical interest?
Anaerobic
- Actinomyces
- Lactobacillus
- Cutibacterium (Proprionibacterium)
- Mobiluncus
- Bifidobacterium
- Eubacterium
- Poststreptococcus
Aerobic
- Listeria
- Nocardia
- Corynebacterium
Which of the non–spore-forming Gram-positive bacteria are anaerobic?
- Actinomyces
- Lactobacillus
- Cutibacterium (Proprionibacterium)
- Mobiluncus
- Bifidobacterium
- Eubacterium
- Poststreptococcus
Which of the non–spore-forming Gram-positive bacteria are aerobic?
- Listeria
- Nocardia
- Corynebacterium
What are the features of Actinomyces?
- Facultative anaerobic or obligate anaerobic
- Gram-positive
- Grow slowly in culture (up to 2 weeks) and produce delicate filamentous forms or hyphae that resemble fungi
- Colonize the upper respiratory tract, upper GI tract, and female genital tract
How are Actinomyces transmitted?
Traumatic implantation or exposure to tissue during surgery
What disease is caused by Actinomyces?
Actinomycosis
What are the features of actinomycosis?
- Development of chronic garnulomatous lesions that become suppurative and form abscesses connected by sinus tracts
- The areas of suppuration are surrounded by fibrotic granulation tissue, giving the overlying surface a hard or woody consistency
- Infections are typically cervicofacial, developing in patients with poor oral hygiene or who have undergone an invasive dental procedure
- Infection can also happen thoracically or abdominopelvically (from abdominal surgery, tuboovarian abscesses, a ruptured appendix, or contaminated IUD)
How is actinomycosis treated?
- Surgical debridement of the involved tissues
- Prolonged administration of antibiotics
How is actinomycosis diagnosed?
Culture. This is difficult as there is often contamination with Actinomyces that are part of the normal bacterial population on mucosal surfaces
What are the features of Nocardia?
- Obligate aerobic rods
- Form branched filaments in tissues and culture, resembling fungi
- Have a Gram-positive cell wall but stain poorly with the Gram stain; they are weakly acid-fast
- Slow growth taking 3–5 days
How are Nocardia transmitted?
- Exogenous infection from soil
- Not normally part of the human microbiota
What is the pathogenesis of infections with Nocardia?
- Pathogenic strains avoid phagocytic killing by secreting catalase and superoxide dismutase
- Nocardiae are able to survive and replicate in macrophages by preventing phagosome–lysosome fusion (mediated by cord factor) and preventing acidification of the phagosome
What are the diseases caused by Nocardia?
- Bronchopulmonary disease
- Cutaneous infections, e.g. mycetoma
- Disseminated brain infection causing brain abscesses
What are the features of bronchopulmonary disease due to Nocardia?
- Slow development, usually in immunocompromised patients
- Cough, dyspnea, and fever are usually present
- Cavitation and spread to the pleura are common
What are the features of cutaneous infections with Nocardia?
- May be primary infections (mycetoma, lymphocutaneous infections, cellulitis, subcutaneous abscesses) or secondary infections arising from a primary pulmonary infection
- Mycetoma is a painless, chronic infection of the feet, characterized by localized subcutaneous swelling with inolvement of the underlying tissues, muscle, and bone; suppuration; and formation of sinus tracts
What are the features of lactobacilli?
- Facultative anaerobic or obligate anaerobic
- Found as part of the normal flora of the mouth, stomach, intestines, and genitourinary tract
Why do lactobacilli rarely caused urinary tract infections?
Lactobacilli cannot grow in urine
In which settings can lactobacilli colonize the blood?
- Transient bacteremia from a genitourinary source (e.g. after childbirth or a gynecologic procedure)
- Endocarditis
- Opportunistic septicemia in immunocompromised patients
What are the features of Cutibacterium (Proprionibacterium)
- Small Gram-positive rods often arranged in short chains or clumps
- Commonly found on the skin, conjunctiva, and external ear, and in the oropharynx and female genital tract
What disease does Cutibacterium acnes (Proprionibacterium acnes) cause?
Acne (in susceptible individuals)
What is the pathogenesis of acne caused by Cutibacterium acnes (Proprionibacterium acnes)?
- Production of a low-molecular-weight peptide by the bacteria residing in sebaceous fillicles attracts leukocytes
- The bacteria are phagocytosed and, after release of bacterial hydrolytic enzymes (lipases, proteases, neuraminidase, and hyaluronidase), stimulate a localized inflammatory response
- The acne is treated through topical application of benozyl peroxide and antibiotics
What diseases do Mobiluncus spp. cause?
Bacterial vaginosis (vaginitis)—M. curtisii
What are the features of Mobiluncus?
- Obligate anaerobic
- Gram-variable or Gram-negative curved rods with tapered ends
Mobiluncus stain poorly with the Gram stain, yet they are considered Gram-positive. Why is this?
- They have a Gram-positive cell wall
- They lack endotoxin (LPS)
- They are susceptible to vancomycin, clindamycin, erythromycin, and ampicillin but resistant to colistin
What are the clinical features of poststreptococci?
- They colonize the oral cavity, GI tract, genitourinary tract, and skin
- They produce infections when they move from these sites to normally sterile sites, e.g. causing sinusitis and pleuropulmonary infections in the upper airways
What are the features of Listeria monocytogenes?
- Short, non-branching, Gram-positive facultative anaerobic rod
- Capable of growing at broad temperature ranges and in a high concentration of salt
- The rods appear singly, in pairs, or in short chains and can thus be mistaken for Streptococcus pneumoniae
- The organisms are motile at room temperature, but less so at 37ºC
- Exhibits weak β-hemolysis
What is the epidemiology of Listeria monocytogenes?
- Human disease is uncommon and restricted to neonates, the elderly, pregnant women, and patients with defective cellular immunity
- The primary source of infection is contaminated food, but vertical transmission is possible
What is the pathogenesis of Listeria monocytogenes?
- L. monocytogenes is a facultative intracellular pathogen
- After ingestion into the GI tract, it survives proteolytic enzymes, stomach acid, and bile salts through stress-response genes
- The bacteria adhere to host cells via the interaction of their cell-surface proteins (e.g. internalin A) with glycoprotein receptors on the host cells (e.g. E-cadherin)
- After penetration into the cells, the acid pH of the phagolysosome activated a bacterial pore-forming cytolysin (listeriolysin O) and two different phospholipase C enzymes, leading to release into the cytosol
- The bacteria proceed to replicate and then move to the cell membrane. This movement is mediated by the protein ActA, which coordinates assembly of actin
- Cellular immunity is most important for this pathogen as it spends most of its time within the cell
What diseases does Listeria monocytogenes cause?
- Neonatal disease
- Disease in pregnant women in the third trimester, when cellular immunity is most impaired
- Mild influenza-like illness in healthy adults
What are the features of neonatal Listeria monocytogenes disease?
- Early-onset disease is acquired transplacentally and can result in abortion, stillbirth, or prematurity
- Late-onset disease is acquired at or soon after birth and manifests as meningitis or meningoencephalitis with septicemia
What are the features of Corynebacterium diphtheriae?
- Possesses medium- and long-chain mycolic acids, but is not acid-fast
- Aerobic or facultatively anaerobic
- Nonmotile and catalase positive
- Ubiquitous in plants and animals, and they normally colonize the skin, upper respiratory tract, GI tract, and genitourinary tract in humans
What is the epidemiology of Corynebacterium diphtheriae?
- Transmission occurs through respiratory droplets or skin contact
- Humans are the only known reservoir
What is the pathogenesis of diphtheria?
- The diphtheria toxin is an A-B exotoxin
- There are three functional regions: a catalytic region on the A subunit, a receptor-binding region on the B subunit, and a translocation region on the B subunit
- The toxin binds to the heparin-binding epidermal growth factor receptor, which is present mostly on heart and nerve cells
- After the toxin attaches to the host cell, the translocation region is inserted into the endosomal membrane, facilitating the movement of the catalytic region into the cell cytosol
- The A subunit then terminates host cell protein synthesis by inactivating elongation factor-2, which is a factor required for the movement of nascent peptide chains on ribosomes
What are the features of respiratory diphtheria?
- The onset is sudden, with malaise, sore throat, exudative pharyngitis, and low-grade fever
- The exudate evolves into a thick pseudomembrane compsoed of bacteria, lymphocytes, plasma cells, fibrin, and dead cells
- The exudate covers the tonsils, uvula, and palate and can extend up into the nasopharynx or down into the larynx
- Diphtheria has become uncommon due to vaccination—there have been no new cases in the US since 2003
