Lecture 5 Bacterial virulence

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Lecture 5 Bacterial virulence pathogenicity

• Aims
• To be able to define and differentiate the terms
• True pathogen
• Opportunistic pathogen
• Commensal microorganism
• To understand what makes bacteria pathogenic
• Reference Black Chp14 p375-388

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Defining some important terms in microbial
pathogenicty

• The term pathogenic means disease causing
• A pathogen is then a disease causing microbe
• There are some further divisions of this capacity
  to cause disease which are useful
• True pathogen
• Opportunistic pathogen
• Commensal

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True pathogen

• A microbe when detected is always associated is
  always associated with disease
• Ie. its detection is always considered
  significant
• eg. Salmonella typhi (typhoid)
• S.aureus
• HIV
• HBV

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Opportunistic pathogen

• A microbe capable of causing disease when
  patients are compromised in some way
• eg HIV, surgery, antimicrobial treatment,
  parenteral feeding, intubation, sedation
• Eg Candida albicans (thrush)
• CMV
• S. epidermidis

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Commensal microbes

• A microbe which is not under normal circumstances
  pathogenic and usually forms a part of the normal
  flora in/on a body site
• However almost any microbe, especially bacteria,
  found in the wrong site (eg by trauma is capable
  of causing infection eg
• E.coli (gut v UT or CSF)
• S. agalactiae (Vag. tract v Neonatal lungs,CSF
  or blood
• and Adult wounds)

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Some other important definitions

• Disease an abnormal disturbance of the bodys
  normal function
• Contamination presence of microbes (can be the
  various grade of pathogens or by commensals
• Infection replication of a grade of pathogen
  generally/eventually resulting in damage (insult
  to the bodys normal function

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What makes bacteria pathogenic?

• The term virulence is used in this context
• Virulence the severity of disease caused by a
  pathogen
• May be related to number of microbes present in a
  site of infection as well as the presence of
  virulence factors

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Virulence factors

• Virulence factors are bacterial attributes which
  help to overcome or avoid the hosts immune
  system so that an infection can be established

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Virulence factors contin...

• Include
• Production of enzymes
• to disrupt cell eg. haemolysin, hyaluronidase
• Clot formation eg. Coagulase
• Antibiotic inactivation eg betalacatmases

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Virulence factors

• Production of structures
• To attach to the host eg. Pili, Flagella,
  capsules
• Avoid immune system eg capsules

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Examples of virulence factors

• Hyaluronidase
• An extracellular enzyme produced by some bacteria
  (eg streps) which allows the bacteria to spread
  into tissue

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Examples of virulence factors contin...

• Coagulase
• Extracellular enzyme causes a clotting of plasma
  (eg S.aureus)
• Does this by converting fibrinogen to fibrin in
  plasma)

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Pathogenic bacteria may contain or produce a
variety of virulence factors

• Exotoxins
• Endotoxins
• Adhesins
• Antibiotic resistance genes
• Capsules
• Immune system avoidance strategies
• eg HIV, HSV,

Capsule
Pili
Genome
Exotoxin
Endotoxin
Plasmid
Cell wall
Flagellum
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The opportunistic pathogen

• Under normal circumstances do not cause or
  associated with disease- but if provided with an
  opportunity can do so-can be lethal
• Compromised hosts include
• HIV (strange infections eg Crypto normal)
• Trauma (include.surgery especially prolonged)
• Invasive devices (urinary IV catheters)
• Immunosuppression (transplant)
• Cytotoxics (eg malignancy)

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Sites where bacterial colonization occurs and
normal flora is found

• Commensal flora
• Found on most mucosal surfaces
• Some cases is transient because of antimicrobial
  body products eg Skin FAs, IgA (secretory
  antibody) lysozyme found in tears, saliva
  mucous
• Can be disrupted eg thrush

• Conjunctiva
• Nasal mucosa
• Oral mucosa
• Pharynx
• Skin
• Colon
• Rectum
• Urethra
• Vagina

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Examples of colonization of the alimentary tract

• In general the commensal are non disease causing
  in the healthy body and if they a contained in
  the site
• Problems occur in cases of overgrowth or invasion
  into other sites eg UTI, trauma, parenteral
  feeding, broad spectrum antibiotics

• Oesophagus/stomach
• Lactobacilli
• Small Intestine
• Enterococci,Lactobacilli, Enterobacteria
• Large intestine
• Enterobacteria, enterococci, bacteroides,
  Bifidobacteria, Eubacteria, Peptococci,
  Peptostreptococci, Clostridia, Lactobacilli