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.