18 - Opportunistic Bacterial Disease Flashcards
True/Primary pathogens
- Cause disease in healthy people
- Relatively more virulent or pathogenic
Examples of primary pathogens
- Bubonic plague
- Cholera
- Anthrax
Opportunistic/secondary pathogens
- Infections that occur when the host immune system is weakened
- Generally low to moderate virulence
1st line of innate immunity
- Skin
- Mucous membranes & secretions
- Normal flora
2nd line of innate immunity
- Innate immune cells
- Inflammation
- Complement
- Antimicrobial substances
3rd line of defence
Adaptive immunity:
- B cells and T cells (helper and killer)
Drug induced disruption to innate immunity
- Antibiotics disrupt normal commensal bacterial flora
- Cancer chemotherapy disrupts immune system
- Anti-rejection medication in organ transplant recipients suppress immune response
Risk factors for opportunistic infections
Age (neonates and elderly), Smoking, Pregnancy, Malnutrition, Obesity, Underlying disease (diabetes)
Characteristics of opportunistic pathogens
- Typically have relatively low pathogenicity
- Usually only cause infection in compromised hosts
Gram negative examples of opportunistic pathogens
- Pseudomonas aeruginosa
- Escherichia coli
- Klebsiella pneumoniae
- Legionella pneumophila
Gram positive examples of opportunistic pathogens
- Staphylococcus species
- Streptococcus pneumoniae
- Clostridioides difficile
- Enterococcus species
Opportunistic pathogen from endogenous source
Derived from normal human flora (e.g. gut bacteria, skin flora)
Opportunistic pathogen from exogenous source
Derived from a transient or chance encounter (e.g. hospital, water, pet)
Device related infections
- Peripheral venous catheters are the most commonly used indwelling device in hospitalised patients
- Device may be contaminated during insertion
- Staphylococcus epidermidis is the leading cause
Staphylococcus epidermidis
- Ubiquitous commensal skin organism
- Relatively low virulence
- Produces only limited toxins
- Ability to cause infection associated with adherence to plastic and production of biofilm
Disruption of skin barrier
- Intact skin is a major defence against infection
- Disruption of the host/environment barrier allows organisms to access deeper tissue or blood
Disruption of commensal flora
- Antibiotics are a major cause of microbiome disruption
- Altered commensal flora allows opportunistic pathogens to flourish
Clostridioides difficile
- Gram positive
- Endospore forming rod
- Strict anaerobe
- motile via flagella
- Ubiquitous in soil & water
- Endospores regarded as major mode of transmission
- Cause of antibiotic associated diarrhoea
Immune disruption
- Medicines can weaken the immune response
(e.g. chemotherapy, corticosteroids, anti-rejection drugs) - Recent infection also alters immune response
Cancer chemotherapy
- Chemotherapy reduces neutrophils (neutropenia)
- Additional risk factor through catheters
- Pathogens include Staphylococci, Streptococci, Enterobacteriaceae and Pseudomonas
Escherichia coli
- Gram negative rods, facultative aerobe
- Member of the Enterobacteriaceae
- Motile by flagella
- Normal component of gut flora
- Endogenous source of infection
- Organism in GI tract can gain access to bloodstream via damaged epithelia
Recent viral infection
- Disrupts the immune response (may also damage epithelia)
- Bacterial pneumonia is a very common, community acquired opportunistic infection
Streptococcus pneumoniae
- Major cause of pneumonia
- Gram positive cocci, facultative aerobe
- Normally founds in pairs, non-motile
- Normal component of upper respiratory tract flora (endogenous source of infection)
- Capsular polysaccharide allows evasion of host immune cells
