Clinical microbiology Flashcards
ILOs
- Have an appreciation of disease mechanisms – entry, establishment, evasion, damage
- Have an understanding of the clinical significance of the anatomy of disease causing microorganisms
Requirements for disease
- Point of entry
- Establishment
- Avoiding host defence mechanisms
- Damaging the host
Portals of entry
Most common hospital infection is UTI
Normally a change in pH in urinary tract (normally acidic) causes UTI
Portals of entry - mucous membranes
Mucous membranes need to be protected:
1.Washing with secretions, e.g. tears,
saliva, mucus (nasal passages) and urine (urethra).
- Filter hairs in nasal passages prevents entry of large particles.
- Cilia in respiratory tract push mucous and microbes upward.
Non-specific innate Mechanisms
Mechanical Factors
Keratinised surface of skin - tough so acts as an
effective barrier against entry i.e., mechanical barrier to microbial to invasion
Mucous membranes
- Form a protective covering that resists penetration and traps many microbes
- are often bathed in antimicrobial secretions which contain a variety of antimicrobial substances
- contain mucosal-associated lymphoid tissue (MALT) (part of the secondary lymphatic system)
Antimicrobial secretions
Many body fluids, e.g. blood, are not normally suitable for microbial growth due to the presence /absence of various factors. e.g.
Normally iron is not bioavailable in blood or breast milk (it is complexed in molecules, e.g. Transferrins or haemoglobin) - bacteria need iron to multiply
Long chains of fatty acids (e.g. oleic acid) occur in slightly acid secretion of the skin (pH 4-6) and these are lethal to many bacteria.
Lactenin - (nitrogenous protein) present in breast milk which are selectively bactericidal for Streptococcus pyogenes - protects against mastitis (inflammation of the mammary gland).
Antimicrobial Secretions - Lysozyme
Main mediator as protection against infection.
Lysozyme hydrolyzes bond connecting sugars in peptidoglycan (cell wall)
Action of lysozyme on the cell wall of Gram+ve bacteria. Peptidoglycan backbone, the β(1-4) bonds connect alternating N-acetylglucosamine and N-acetylmuramic acid (NAM) residues.
Action of lysozyme is more effective on G+ve bacteria as the peptidoglycan cell wall is more easily accessed (phospholipid bilayer somewhat protects a G-ve bacterium)
Gastrointestinal tract
• stomach
– gastric acid
• intestines
– pancreaticenzymes– bile
– intestinal enzymes– GALT (M cell patches - Peyers patch)
– peristalsis
. intestines
– shedding of columnar epithelial cells
– secretoryIgA
– normal microbiota– Paneth cells
- produce lysozyme
- produce cryptins (defensins: peptides)
Genitourinary tract
low pH of urine and vaginal epithelia
urea and other toxic metabolic end products in urine hypertonic nature of kidney medulla
flushing with urine and mucus
Antimicrobial Peptides - Defensins and cationic peptides
Defensins:
- Defend from pathogens
- Shape Microbiota
- Protect stem cells
peptides that are open- ended, rich in arginine and cysteine, and disulfide linked. Found in neutrophils, intestinal Paneth cells and intestinal and respiratory epithelial cells
Cationic peptides
e. g. cathelicidin, produced by a variety of cells
e. g. neutrophils and respiratory epithelial cells
LL37 (leucine leucine 37 a.a.)
- proteolytic product of a cathelicidin
- amphipathic
Antimicrobial Peptides - Bacteriocins
Bacteriocins
Peptides produced by bacteria including normal microbiota
Lethal to closely related species
e. g., colicins produced by E. coli
e. g., sakacins produced by lactobacilli
Antimicrobial Peptides - Selectivity
- host mammalian amphipathic antimicrobial peptides selectively bind to the differential membranes
- host secretions are amphipathic so really good at binding to bacterial membranes (cholesterol molecule in host membrane decreases charge on the membrane, bacteria do not have this, and are much more negatively charged)
- damage the integrity of the cell they bind to
Establishment
Adherence factors associated with establishment of infection
Adherence mediation: nearly always proteins
Adherence factors associated with establishment of infection - N. meningiditis
- interactions are multi-adhesion, multi-molcule
- proteins are predominantly involved in attachement
- through pili and pores
Evading host defences
• Leukocidins
– Kill white blood cells including neutrophils and macrophages!
Produced by Staphylococcal and Streptococcal spp.
• Coagulase
– Causes fibrin clots to form in the blood of a host. Advantageous to the bacteria in evasion
Damage (exotoxins)
- Neurotoxins – cause paralysis
- Enterotoxins – cause sickness and diarrhoea
- Cytotoxins – cause cell death
Damage
- classic damage by TOXINS
- EXOTOXINS (secreted)
- EXOTOXINS (LPS, lipopolysaccharide on the SURFACE of G-VE bacteria)
TB has no obvious virulence factor (toxin that causes damage in absence of the bacterial cell)
Damage - comparison between exotoxins and endotoxins
LPS = endotoxin, part of the cell
Highly toxic
Bacterial anatomy
Certain bacterial structures satisfy several of the requirements for satisfactory infection.
The clinical significance of bacterial anatomy:
- Structures outside the bacterial cell wall - cause the most disease
- The bacterial cell wall
- Inside the cell wall
Bacterial anatomy - G+ve vs G-ve
Bacterial anatomy: clinical significance of the bacterial cell wall
Gram Positive
• Gram positive:
– Teichoic acid is being considered as a vaccine target for both S. aureus and C. difficiledue to the high levels of antibiotic resistance.
– M protein from Strep. pyogenesis highly variable but important in infection- protrudes from cell wall
– Mycolic acid from Mycobacterium tuberculosis prevents the action of many antibiotics and host effects (‘myco’ = Greek for slimy, slimy because of mycolic acid) unique to actinomycetes
Bacterial anatomy: clinical significance of the bacterial cell wall
Gram Negative
Gram negative:
- Lipid A anchors LPS to the outer phosholipid bilayer. Release of this leads to a heightened immune reaction. Endotoxin!
- O-antigen highly variable and recognized by the immune system. Can be used in typing (e.g. Ecoli 0157)
Bacterial anatomy: structures outside the cell wall: Glycocalyx
Structures outside the bacterial cell wall - called Glycocalyx
– Polysaccharides and proteins. If loosely attached referred to as slime layer. If they form a highly organized structure more commonly known as Capsule. (poly-d-glutamic acid)
– Capsule is important in preventing phagocytosis and allowing the infection process to continue
Bacterial anatomy: Fimbriae and Pili
Structures outside the bacterial cell wall.
• Fimbriae and pili
– Involved in adherence. Bundle forming pili from E. coli.
– Shorter than flagella and typically
found on Gram negative bacteria.
– Also aids in motility such as gliding or twitching motility
– Potential vaccine candidates.
– Pili also used in immune evasion through a process of antigenic variation.
