Lecture 4 Introduction to microbial pathogens Flashcards
What is a pathogenic microorganism?
Agent capable of causing disease in a host
What is a disease?
Disorder of structure or function producing symtoms and signs
Example of it being possible to be infected by pathogen without disease being produced:
Pathogen in NW of Aus called Murray Valley encephalitis (MVE) but only 1:800-1000 of infected people develop significant disease
What types of microorganisms can be found on host?
Commensal
Pathogen
Opportunistic pathogen
What are commensals?
Bacteria primarily
Colonise many body sites
Referred to as commensal flora/normal flora or microbiome
How much of faeces are bacteria?
60% of dry weight
What is lactobacillus useful for?
Maintains vaginal pH at 3.8 - 4.5 this pH protects from other harmful bacteria.
Are commensals all over the body?
No they are in heavily colonized body sites (such as the oral cavity, oropharynx, nasopharynx, bowel, vagina, skin)
They are not present on sterile body sites (Bloodstream, cerebrospinal fluid, bladder, peritoneal space, and joint space)
What are the types of sources that pathogens can come from?
Exogenous sources (external source)
Endogenous sources (microbiome)
What is the carrier state of commensals?
When pathogens are found in normal flora without causing disease
What is another way of carrying disease?
Some pathogens can remain carried for months after recovery from disease
Salmonella typhi
Can asymptomatic carriers be a source of infection of others?
Yes
What are opportunistic pathogens?
Pathogens very low in virulence so don’t cause disease
Opportunistic diseases cause infection when they can bypass the host’s innate immune defences.
What is toxoplasma gondii?
Protozoan parasite definitively hosted by cats and can infect humans and other mammals
How is toxoplasma gondii infection acquired?
Uncooked meat or cat faeces
Is infection of toxoplasma gondii common in humans?
Yes with 15 - 70% of people infected but it is asymptomatic
How does toxoplasma gondii affect humans?
It remains dormant in various tissues in what are known as pseudocysts.
If immune system fails the cysts can become active and cause damage to brain and other host tissue
What is candida albicans?
A fungal yeast that colonizes humans and causes diseases like vaginal thrush
What are the harms associated with candida albicans?
It can bypass innate defences when immunocompromised and infect many internal organs
How can people be susceptible to candida albicans?
Intravenous cannulas
Broad spectrum antibiotics
Urinary catheters
Surgical procedures
Corticosteroid therapy
Neutropenia
Severe burns
Parenteral nutrition
Superficial infection elsewhere
What are virulence factors?
Properties that microbes have that enable them to cause disease.
Disease can be caused by the microbe or the host’s reaction to it
What is a good example of a virulence factor?
Cholera toxin enables vibrio cholera to cause disease
How does cholera toxin kill people?
Causes lots of fluid loss and hypotension
What types of organisms can be pathogenic?
Bacteria (unicellular prokaryotes that can live and replicate outside the host)
Viruses (not cellular and can only replicate in living cells)
Fungi are eukoryotic multicellular organisms that can live or replicate outside of host
Protozoa are unicellular eukaryotes that can live and replicate outside of human host
Helminths are non0microorganisms with complex life cycles within or without host (eg. gineaworm)
Pions non organisms that are just proteins that cause disease
What system of classification is important with bacteria?
Genus and species
What does the name tell doctors?
Specific disease potential
How it was acquired
How it can be transmitted
The drugs that are likely to effectively treat it
What differences can we see within species?
Population
Strains
Clones
Genotypes
Serotypes, serogroups
What does the cytoplasmic membrane of a bacterium typically consist of?
Phospholipid bilayer
An osmotic barrier
Performs organelle like functions such as active transport of nutrients and ions
Synthesis of lipids, peptidoglycan precursors, lipopolysaccharides, and capsular polysaccharide
Protein secretion
Electron transport
Flagellar movement and chemotaxis
What does the nucleoid consist of?
single continuous DNA double strand
What do both prokaryotes and eukaryotes contain?
ribosomes (E.coli can have 1500 per cell)
What are plasmids?
circular DNA that can be transferred between cells
Where are genes for virulence factors and antibiotic resistance typically contained?
Genes for virulence factors
Antibiotic resistance
What is the cell wall made up of?
crosslinked peptidoglycan
What does cell wall do?
Provides rigidity to bacterial cells
What enzymes create cross linkages?
Transpeptidases (if this is interfered with we have no cell wall = easy to burst bacterium)
What organisms posses an outer membrane?
Only gram negative-bacteria
What is the function of the outer membrane?
Main function is osmoregulation and contains important embedded molecules such as Lipopolysaccharide (virulence factor causing toxic shock) LPS is antigenic
What are some proteins that are found in bacterial outer membranes?
Porin proteins (trimeric proteins that control ingress and egress of molecules including antibiotics)
Outer membrane proteins (OMPs) with numerous important functions such as survival in macrophages.
What do OMPs do?
Survival in macrophages
Resistance to complement
Attachment to host cells
Efflux pumps
Iron acquisitions
What is the capsule “slime layer”?
Present in some bacteria
Usually made of polysaccharide
What does the capsule do?
Antiphagocytic
Adhesin function
Antigen
Component of biofilm
What is the implication of the adhesivity of the capsule on prosthetics?
If there is any sign of infection the prosthetics must be removed
What are fimbriae?
Hair-like projections that are also important for horizontal gene transfer.
What are flagella?
Whip-like structures that confer motility (possible virulence factor)
Are flagella always the same arrangement?
No different arrangements can be often seen
What is the difference between gram positive and gram negative bacteria?
Gram positive: Purple when stained with crystal violet due to presence of
thick cell wall
Gram negative:
Stains red when dyed with crystal violet due to presence of thin cell wall
Has outer membrane
Has thinner cell wall
How is a gram stain done?
Specimen from patient added to glass slide then flooded with crystal violet which gets among peptidoglycan cell wall
The slide is then flooded with Lugol’s iodine for 30 seconds. Iodine fixes crystal violet onto peptidoglycan.
The slide is then rinsed with H2O.
Crystal violet is then decolourized with the alcohol in everything but gram positive bacteria. Thick cell wall retain the crystal violet but thin wall don’t.
The slide is then flooded with carbol fuchsin counterstain to show the invisible bacteria that was decolourised as a result of lower retention of crystal violet
What are the types of morphology we can observe in bacteria?
Cocci (round balls)
Bacilli (rods)
Comma shaped (vibrio)
Helical (campylobacter)
Spirochaetes
What are the types of arrangements observed in coccus bacteria?
clumps
Chains
diplococcous
What are clumped balls typically?
Staphylococcus
What are stringed balls typically?
Streptococcus
What are diplococci?
double ball shape
What is irregular arrangement called?
diptheroids or coryneforms
Which bacteria do not take up gram stain?
spirochaetes and mycobacterium
What is done to bacteria that don’t take up gram stains?
A special stain is used for them. Acid-fast stain or ziehl-Neelsen stain
What is the clinical significance of the gram stain?
It is the first piece of information clinicians gather about bacteria
It allows early management decisions
What information is achieved from a gram stain?
Whether it is gram negative or positive
Colour
Morphology
Summary
Not all microorganisms found on the body are pathogens
Not all pathogens cause disease
The gram stain appearance (colour and morphology) of a bacterium is very important.
Name of microorganism carries important information
Name an example of a gram-negative curved or comma shaped rods called?
Vibrio
Name a gram negative helical rod:
Campylobacter
Name a gram positive filamentous or branching rod:
Nocardia
