1.19.5.11 - Commensals of the Respiratory Tract
Microbiota: the combined microorganisms in a particular environment
Vast majority of resident bacteria are not harmful to the host
number of bacteria ~ number of host cells
Mammals are host to a vast number of microorganisms,
commensals
Harmonious co-existence with host in appropriate environment
They do have a potential to be opportunistic pathogens
History of co-evolution with the host
beneficial impact of commensals
can compete again pathogens. Space colonized by commensals is not available to pathogens
play a role in nutrition, eg the synthesis of vitamin K (covered in GIl module Y2)
can stimulate an immune response and support development of immunity
sites
rumen
superficial reproduction organs
skin, hoof, hair
intestinal tract
oral cavity
upper resp. tract
upper resp tract
highly colonised
Commensal organisms colonise epithelia surfaces
Cilia and secretions prevent excessive accumulation of organisms
distal airways are kept sterile by:
physical barriers
immune cells
non-specific soluble factors
lactoferrin
complement
lysozymes
surfactants
defensins
reduce the particles that get to the lower resp tract
hairs and structures of nasal turbinates
movement of cilia and mucus in the trachea (mucociliary clearance)
macrophages
lungs
In practice the defence mechanisms are very good and keep things in check
Respiratory disease is rare unless the system is impaired or challenged with an specific pathogen
Very low numbers of bacteria in the healthy lungs
clila prevent a build up of mucus
commensal vs pathogen
acquisition of virulence genes
change in gene expression
location on host
host specific
Example: Bite wound: Bacteria that colonise the oral cavity as commensals turn opportunistic pathogens when they are in the blood stream if that epithelial barrier breaks
Example: Changes in surface capsule changes ability of macrophages to phagocyte bacteria and control bacterial growth
host controls impaired
Co-infection with other pathogen (Intercurrent disease)
Environment related
Changes within the host, e.g. pregnancy, stress, reduced maternal antibodies in neonates.
All allowing an opportunity for infection through reduce immunity or other controls.
Example: Dehydration or cold can increase mucus viscosity. This will slow the escalator and reduce function in clearing commensals that make it that far.
Example: Infection with a virus can damage the mucosal function allowing overgrowth of commensal bacteria.
risk factors
development of pneumonia
calves
close confinement
transportation and stress
poor ventilation
overhousing
decline in maternal antibody
intercurrent infections
carriage vs commensalism
Carriage: A pathogen is carried by a convalescent or unaffected host without clinical signs
Carriage is can be central to maintaining disease in populations.
A host immune response may control a pathogen but not clear it.
A host animal may be resistant.
can lead to a carrier state
EXAMPLE: Carriage in URT: Strangles
Streptococcus equi (cause of Strangles)
Recommended if carrier for more than 2 months to treat site of infection
Screening a nasal pharyngeal swab may not pick it up (recommended three consecutive swabs).
S. equi found to reside in the guttural pouch
Animals found to carry up to a month post clinical signs and can spread disease
Roughly 24% of ‘recovered’ animals can shed S. equi.
microbiota considered commensals
change in organism
change to hose or location on host
pathogens (organisms which actively casue disease
opportunistic pathogens
can cause infections when given the chance
may live on host without disease and require secondary conditions for infection
common bacteria of the healthy upper resp tract
Pasteurella multocida
Actinobacillus
Bordetella bronchiseptica
Streptococcus species.
Escherichia coli
gram positive cocci in chains
fastidious requrie enriched media blood agar
facultative anaerobes
catalase negative, usually haemolytic
commensals of mucous membranes
susceptible to dessciation
cause pyogenic infection
variable haemolyisis depending on species
disease - strangles
non motile, urease positive, oxidase positive, agglutination of red blood cells
facultative anaerobes
non enriched media, most grow on macconkey
most are commensals of the upper mucous membranes andcause a wide range of disease conditions
gram negative rods
disease - Actinobacillus pleuropneumonia causes pleuropneumonia in pigs
facultative anaerobes
non motile, oxidaase positive
enriched media
Some strains form mucoid colonies due to thick hyaluronic acid capsules
small gram negative rods
disease
Phase variation
Commensals of the upper respiratory tracts but can also cause opportunistic respiratory disease
changing the serotype of the bacteria lead to increased colonisation and lead to disease (see notes on phase variation slide)
Shipping fever/pneumonic pasteurellosis in cattle.
Snuffles in rabbits.
motile, catalase positive, oxidsae positive, agglinatation ofred blod cells
These are commensals of the upper respiratory tract, and cause opportunistic infections
obligate aerobes
Diagnosis from nasal swabs, tracheal aspirates and exudates
non enriched media
Bordetella exhibit phase changes which correlate with virulence
small gram negative rods
A tracheal cytotoxin inhibits ciliary motility
disease - Respiratory disease in a wide range of mammals, dogs (kennel cough), pigs, kittens, horses, rabbits, hedge hogs)
motile, catalase positive, oxidase negative, agglutination or red blood cells
Opportunistic infections of respiratory tract
facultative anaerobes
Common enteric commensals of the GI tract Some species are pathogens containing specific virulence factors
ferment lactose - macconkey
More common primary cause of pneumonia or mixed infections in shelters
non enriched media
disease- Avian colibacillosis – aerosacculitis (& systemic disease) in poultry
gram negative rods
diagnostic approach
swabs
start point
sampling
visual/imaging
cytology/microbiology on samples
radiography
ultrasonography?
endoscopy/rhinoscopy
clinical signs
clinical exam
history
bronchoalveolar lavage
tracheal aspiration
haematology
swabs
demonstration of immune cells
demonstration of viral antigens
culture or visual - bactrieral or fungal
swabs have telescopic sleeves that prevent contamination of the swab before site of interest
culture of organisms may be mixed commensals
bronchoalveolar lavage
bacterial culture
microscopy
adequate restraint is essential for this procedure
froth on sample is due to surfactant
specific fluid in the bronchi and then collect
can demonstrate intracellular bacteria BAL
some neutrophils contain intracytoplasmic bacteria
large numbers of neutrophils
outcomes are not always conclusive
outcomes
not always conclusive
bacteria exist in mixed populations
many present whehther there is disease or not
could be viral
targeting sites of infection can improve identification
combination of culture and PCR
infection control
medical prevention
husbandry
prebiotics and probiotics
prophylactic treatment - problems with antibiotic resistance (not recommended)
vaccines
breeding
biosecurity
reduce stress
Modify conditions where disease may spread more easily.
Improve health and therefore resistance to disease.
summary
In healthy animals, the URT is highly colonized with commensal organisms, in contrast to the LRT, which has a very low level of commensals, generally below the limit of detection of culture based methods.
Carriage is different to commensalism, A pathogen is carried by a convalescent or unaffected host without clinical signs
Commensal are beneficial to the host as they are essential to develop the immune system, compete against pathogens & play a role in nutrition.
Examples of commensals of the heaty URT include, Streptococcus sp, Actinobacillus sp, Pasteurella multocida, Bordetella bronchiseptica and E. coli.
Commensal microorganisms harmoniously co-exist with the host but can turn into opportunistic pathogens under certain environmental & host conditions.
Sampling for bacterial culture needs to target the site of infections. Interpretation needs to be with care as often mixed cultures.