pathology

• Less than 100 human pathogenic species identified and >5000 bacterial species identified to date (estimated many times more than this on Earth). Therefore < than 0.02%, but probably <0.0001% of bacterial species are human pathogens.
• Why do bacteria cause infections?

  Body provides favourable niche in which to reproduce; human tissues provide nutrients and reduced competition for resources
• Why don't all bacteria cause infections?

  • Not favourable environment for all species
  • Immune system prevents vast majority of bacteria from colonizing, pathogens need adaptations to avoid or overcome immune response
• Where can bacteria infect?

  • Gastrointestinal tract
  • Central nervous system
  • Respiratory tract
  • Urogenital tract
  • Skin and muscle
  • Multisystem
• Gastrointestinal tract


  • Escherichia coli
  • Shigella
  • Salmonella
  • Campylobacter jejuni
  • Vibrio cholerae
  • Clostridium difficile
• Central nervous system


  • Neisseria meningitidis
  • Haemophilus influenzae
  • Streptococcus pneumoniae
  • Clostridium tetani
  • Clostridium botulinum
• Respiratory tract


  • Streptococcus pyogenes
  • Streptococcus pneumoniae
  • Bordetella pertussis
  • Mycobacterium tuberculosis
  • Legionella pneumophila
  • Pseudomonas aeruginosa
  • Haemophilus influenzae
  • Corynebacterium diphtheriae
• Urogenital tract


  • Escherichia coli
  • Proteus mirabilis
  • Chlamydia trachomatis
  • Treponema pallidum
  • Neisseria gonorrhoeae
• Skin and muscle


  • Staphylococcus aureus
  • Streptococcus pyogenes
  • Rickettsia prowazekii
• Multisystem


  • Borrelia burgdorferi
  • Yersinia pestis
  • Franciscella tularensis
  • Coxiella burnetii
  • Leptospira spp.
• Ingestion (faecal-oral)


  • Infection - Pathogen reaches gastrointestinal tract and rapidly multiplies e.g Campylobacter jejuni, Salmonella species (S. enterica s. Typhimurium - typhoid), certain strains of Escherichia coli
  • Intoxication - Ingestion of toxins produced by bacteria. Typically results in acute vomiting 1-6 h post ingestion e.g. Clostridium botulinum
• Vector-borne


  • Ticks - Lone Star tick - Ehrlichia chaffeensis - Ehrlichiosis
  • Ticks - Ixodes tick - Borrelia burgdorferi - Lyme disease
  • Ticks - Many ticks - Coxiella burnetii - Q fever
  • Ticks - Wood tick/Dog tick – Rickettsia rickettsii – Rocky Mountain Spotted Fever
  • Lice - Body louse – Rickettsia prowazekii – Epidemic typhus
  • Fleas - Rat fleas – Rickettsia typhi – Endemic typhus
  • Fleas - Rat fleas – Yersinia pestis – Bubonic plague (pneumonic is airborne)
• Zeihl-Neelsen stain of skin biopsy reveals Mycobacterium leprae - bacteria are stained red/pink
• Pathophysiology


  • Virulence factors enable pathogenic bacteria to colonise, invade and replicate within host organism
  • E.g. adhesins and fimbriae, capsule, exoenzymes, toxins
  • Some of these enable the bacteria to avoid or overcome the immune response
  • Some bacteria produce toxins that help to overcome immune response and enable them to invade cells / access nutrients cause direct damage to host human cells / tissues / organs
  • In many cases damage caused during bacterial infection is the result of immune responses to the pathogen and not directly due to the pathogen itself
• Prevention of bacterial infections

  • Hygiene & sanitation - Personal, Food, Environment (e.g. water treatment)
  • Vaccines: Inactivated, attenuated, subunit, toxoid - DTaP (Diptheria, Tetanus, Pertussis), Hib/MenC & MenACWY (Meningitis)
  • Prophylaxis (antibiotics, immunotherapy, antimicrobial coatings) - Post surgery, In pregnancy if GBS carriage detected, Immunocompromised e.g. Sickle cell anaemia, Antimicrobial coatings on medical devices
  • Safe sex
  • Insecticides (vector control)
  • Quarantine and contact tracing
• Detection


  How do you diagnose a bacterial infection?
• Detection of bacterial infections

  • Patient presentation (signs and symptoms) - E.g. fever, diarrhoea, vomiting, cough, pus/exudate, rash
  • Identify most likely causative agents
  • Samples taken from sputum, blood, urine, stools
  • Urine dip-stick test
  • Microbial culture - Colony morphology, selective media
  • Microscopy - Cellular morphology, Differential stain
  • Biochemical tests
  • Molecular diagnostics e.g. PCR
  • Serological diagnostics e.g. ELISA
• Treatment


  How do you cure bacterial infections?
• Treatment of bacterial infections

  • Topical disinfectants - Chlorhexidine
  • Surgery / Debridement
  • Antimicrobial coatings - Silver nanoparticles used in wound dressings
  • Antibiotics - Target specific components of bacterial cellular machinery, Inhibit growth = bacteriostatic, Cause bacterial death = bactericidal, Oral / intravenous / intramuscular / topical, Usually prescribed a course of treatment
• How do antibiotics work?

  • Combination of antibiotics may help prevent evolution of resistance and treat infections caused by MDR bacteria
  • Clavulanic acid given in combination with penicillin to inactivate B-lactamases
• Concern: Antibiotic resistance
• Fix the antibiotics pipeline M. A. Cooper & D. Shlaes Nature 472, 32 (2011)
• Case study 1: Airborne


  Mycobacterium tuberculosis - Causative agent of Tuberculosis
• Mycobacterium tuberculosis


  • Weak Gram positive rod, discovered by Robert Koch (1882)
  • Determined that this is causative agent of the disease tuberculosis (Koch's postulates)
  • Mycobacteria have a unique cell wall structure with high concentration of mycolic acid
  • Slow growing (generation time approx. 16 hr) – takes 10 – 14 days to culture in lab
• Tuberculosis: infection vs. disease


  • Tuberculosis (TB) is an ancient human disease
  • Clear cut distinction between infection & disease
  • only 5% of infections lead to disease, 95% latent infections
  • If untreated 50% active infections are fatal
  • M. tuberculosis survives and multiplies in macrophages
• Rook et al. (2005) Nature Reviews Immunology 5, 661-667
• Tuberculosis: Latent infection


  • Immune response involves formation of granulomas ("tubercles") consisting of macrophages, lymphocytes and epithelioid cells which trap the bacteria
  • Mtb survive inside granulomas and become dormant, but can be reactivated to cause active infection if untreated
• Tuberculosis: Active disease


  • Fever
  • Night Sweats
  • Weight loss
  • Cough
  • Cavities in lung
  • 85% of cases in the lungs (pulmonary TB) but other organs can be affected (extrapulmonary TB) e.g. lymphatics, kidneys, urogenital tract, CNS, bones, blood vessels (death by hemorrhage)
  • Fatality in 50% untreated cases - occurs due to aggravated immune response
• Tuberculosis: Detection


  • Use Ziehl Neelsen stain to differentiate acid-fast bacteria
  • Sample stained with carbolfuschin stain and heated – all cells stain red/pink
  • After washing with alcohol….Acid fast bacteria retain the carbolfuschin stain due to high concentration of mycolic acids in cell wall and they remain red/pink
  • Non-acid fast bacteria are decolourised as alcohol can penetrate the cell wall due to lower lipid concentration and they are then counterstained with methylene blue
• Tuberculosis: Detection


  • Mantoux test - 0.1 ml of PPD injected into the forearm to produce a small bleb, Localised hardening ("induration") indicates hypersensitivity +ve test indicates infection or previous infection/vaccination
  • X-Ray test - Cavities seen as radio-opaque patches in lower parts of lung
• Tuberculosis: Treatment/Prevention


  • Triple therapy over 6 months of:
  • Isoniazid - interferes with mycolic acid cell wall synthesis
  • Pyrazinamide - action unknown, but converted to active form in the liver, pyrazinoic acid
  • Rifampicin / Rifampin - inhibits transcription by binding to β-subunit of RNA polymerase
  • Alarming increase in rifampicin resistant (RR) and multi-drug resistant (MDR) strains
  • Prophylaxis for 1 year with Isoniazid recommended for people in close contact
  • BCG (Bacille Calmette-Guérin) vaccine - live attenuated strain of M. bovis; 20-80% efficacy
• Tuberculosis: Epidemiology


  • TB is in the TOP 10 leading causes of death globally
  • It is the leading cause of death
• Non-acid fast bacteria


  Decolourised as alcohol can penetrate the cell wall due to lower lipid concentration and they are then counterstained with methylene blue
• Acid fast bacteria


  Have a high concentration of mycolic acids in cell wall and they remain red/pink
• Tuberculosis: Detection


  1. Mantoux test - 0.1 ml of PPD injected into the forearm to produce a small bleb

  2. Localised hardening ("induration") indicates hypersensitivity

  3. +ve test indicates infection or previous infection/vaccination

  4. X-Ray test - Cavities seen as radio-opaque patches in lower parts of lung
• Tuberculosis: Treatment/Prevention


  1. Triple therapy over 6 months of:

  2. Isoniazid - interferes with mycolic acid cell wall synthesis

  3. Pyrazinamide - action unknown, but converted to active form in the liver, pyrazinoic acid

  4. Rifampicin / Rifampin - inhibits transcription by binding to β-subunit of RNA polymerase

  5. Prophylaxis for 1 year with Isoniazid recommended for people in close contact

  6. BCG (Bacille Calmette-Guérin) vaccine - live attenuated strain of M. bovis; 20-80% efficacy
• Alarming increase in rifampicin resistant (RR) and multi-drug resistant (MDR) strains
• Tuberculosis is in the TOP 10 leading causes of death globally
• It is the leading cause of death from a single infectious agent
• 2 billion people currently infected globally (latent TB, do not transmit)