Types of Microbes Flashcards
What are some specific investigations done when a bacterial infection is suspected?
Microscopy e.g. Gram stain, CSF Culture Antibiotic susceptibility Antigen detection Nucleic acid detection
Name some supportive investigations done when infection is suspected.
FBC = neutrophils, lymphocytes (indicates inflammatory response)
CRP = acute phase proteins which have been stimulated by non-specific inflammation
Liver & kidney function
Imaging e.g. X-ray, ultrasound, MRI
Histopathology
What are some specific investigations needed when a viral infection is suspected?
Antigen detection
Antibody detection (ELISA)
Viral nucleic acid detection
Give some examples of DNA viruses.
single-stranded, non-enveloped: Parvovirus 19 (foetal defects)
double-stranded, non-enveloped:
- adenovirus (respiratory, gastroenteritis, conjunctivitis)
- BK virus (lies dormant in kidneys; affects immunosuppressed)
- HPV
- JC (causes progressive multifocal leucoencephalopathy in AIDS patients)
double-stranded, enveloped:
- herpes
- hepatitis B (liver cancer)
- Molluscum contagiosum (water warts; increased incidence in children, sexually active adults, and AIDS patients)
Give some examples of RNA viruses.
single-stranded, positive, non-enveloped:
- enterovirus (viral meningitis, polio, coxsachievirus, echovirus)
- hepatitis A & E
- norovirus
single-stranded, positive, enveloped:
- HIV
- hepatitis C
single-stranded, negative, enveloped:
- ebola
- measles
- mumps
- influenza
- respiratory syncytial virus
double-stranded, non-enveloped: rotavirus (diarrhoea)
Give some examples of clinical problems caused by commensals being displaced.
Epithelium breach e.g. burns, surgery, drug uses, IV lines
Faecal-oral route
Faecal-perineal-urethral route e.g. UTIs (women)
Poor dental hygiene/dental work
RISK!: in asplenic/hyposplenic patients (loss of splenic macrophages so IgG & C3b-bound bacteria are not phagocytosed), damaged/prosthetic valves, previous infective endocarditis
Give some examples of clinical problems caused by the overgrowth of commensals.
Immunocompromised:
- diabetes (reduced immune system & increased blood glucose)
- AIDS
- malignant disease
- chemotherapy
Antibiotic depletion
- thrush (Candida albicans)
- colitis (Clostridium difficile)
How do immune cells recognise pathogens?
Pathogen-associated molecular patterns (PAMPs)
e.g. carbohydrates, lipids, proteins, nucleic acids
Pathogen recognition receptors (PRRs)
e.g. Toll-like receptors
Opsonisation = coating microbes with opsonins -> increased attachment of phagocytes & clearance of microbes
e.g. complement proteins, antibodies, acute phase proteins (liver - CRP & mannose-binding lectin)
Note: opsonisation is essential in clearing encapsulated bacteria e.g. meningitis causative organisms
Outline how infection leads to multi-organ failure in the context of the immune system.
Infection -> Endo/exotoxin -> Overreaction of neutrophils, endothelium monocytes (TLR4 receptors & complement) -> Excessive systemic inflammatory response (cytokine shower, coagulopathy, vasodilatation, and capillary leak - reduced tissue perfusion) -> Sepsis -> Organ failure
Name some commensal skin microbiota.
VIRUSES:
- papilloma (hand/genital warts)
- Herpes simplex (cold sores/genital herpes)
BACTERIA:
Gram +ve:
- Staph. aureus
- Coagulase -ve Staphylococci (non-haemolytic)
- Corynebacterium (common contaminant of blood cultures - Gram-ve rods)
Gram -ve: Enterobacteriaceae (skin below waist - hygiene)
FUNGI:
- yeasts: Candida
- dermatophytes: Athlete’s foot, hair loss, rashes
PARASITES:
- mites (eyelashes)
- scabies
Name some mucosal flora commensals.
EYE:
- Coagulase -ve Staph.
- saprophytic Neisseria spp. (non-pathogenic unless eye is penetrated)
- viridians group Strep. (alpha-haemolytic: digests blood -> green pigment)
NARES: Staph. aureus (swab site for MRSA), Corynebacterium
NASOPHARYNX: (meningitis-carrying organisms)
- Neisseria meningitidis
- Strep. pneumoniae
- Haemophilus influenzae
MOUTH: viridians Strep. —> endocarditis
STOMACH:
- Helicobacter pylori
- Strep.
- Staph.
- Lactobacilli
INTESTINE: (Gram-ve organisms)
- Lactobacilli
- Coliforms
- Strep.
- Clostridium
URETHRA: (moves up urethra -> cystitis, polynephritis)
- Enterobacteriaceae
- Lactobacilli
- Strep.
- Enterococci
note: take midstream sample to avoid false +ve
VAGINA:
- Lactobacilli
- Group B Strep. (can cause septicaemia & meningitis in babies delivered vaginally)
- yeasts (-> thrush after antibiotics)
Give some examples of infections caused by commensal bacteria.
INVASION: Strep. pyogenes -> pharyngitis
MIGRATION: E.coli -> UTI
INOCULATION: Coagulase -ve Staph. -> prosthetic joint infection
HAEMATOGENOUS: viridians Strep. -> endocarditis
Give some examples of internal and external natural surface infections.
EXTERNAL: cellulitis, pharyngitis, conjunctivitis, gastroenteritis, UTI, pneumonia
INTERNAL: endovascular (endocarditis, vasculitis -> mycotic aneurysm - aneurysm arising from bacterial infection of arterial wall), septic arthritis, osteomyelitis, empyema (pleural cavity infection)
Give some examples of prosthetic surface infections.
Intravascular lines e.g. Hickman line (central venous catheter)
Peritoneal dialysis catheters -> continuous ambulatory peritoneal dialysis infection
Prosthetic joints (Coagulase -ve Staph. & Staph. aureus)
Urethral catheters -> catheter-associated UTIs (CAUTIs)
Cardiac valves/pacing wires/endovascular grafts (endocarditis)
Ventriculo-peritoneal shunt (drains excess fluid around brain into peritoneum where it is absorbed) -> ventriculitis (meningitis)
Name the causes of endocarditis.
Natural:
- heart failure/MI -> turbulent flow damages valve endothelium
- bicuspid aortic valve
Prosthetic: colonisation of endothelium by bacteria -> vegetation formation (biofilm) -> embolism/mycotic aneurysm
> 1yr post-op = viridians Strep., Enterococcus faecalis, Staph. aureus (IV drug abuse)
<1yr post-op = coagulase -ve Staph.
