Infectious Diseases Week 3

What are the three components of the epidemiologic triad

Environment, Host, and Agent

what is the environment

External factors that affecting agent and the opportunity for exposure

These are physical, social, economic, and biological factors that facilitate of act as a barrier for transmission of infectious diseases to a host

what are examples of the environment

Climate, sanitation, living conditions, and healthcare infrastructure

what is the host

The organism susceptible to an infectious disease

what are Factors influencing infection

genetic susceptibility, immune status, age, and behavior of the host

what is the agent

Causative factor of an infectious disease

eg. Bacterium, virus, parasite

what is Symbiosis

Together-life" with a host

What are ectosymbionts

Symbionts found on the surface of a larger organism

What are endosymbionts

Symbionts found within an organism

what are the types of symbiotic relationships

intermittent, cyclic, or permanent

What are examples of transient associations in disease

Listeriosis, malaria, leptospirosis, legionellosis, vaginosis

What happens in permanent associations between host and symbiont

They co-evolve

What is a consortium

A host with more than one symbiont

Do symbiotic interactions only affect the host and symbiont

No. Interactions impact the larger biotic community and wider ecosystem

what are the types of beneficial relationships

mutualism

cooperation or synergism

commensalism

What is mutualism

Reciprocal benefit between obligatory, dependent organisms

They may not survive as individual organisms

what are examples of mutualism

microbe-insect, methane-based, rumen-microbe

What is cooperation or synergism

A syntrophic relationship where growth relies on or is improved by nutrients from another in proximity

They can be separate but may hamper their functions

what is an example of cooperation

beneficial metabolites produced by gut bacteria

What is commensalism

A unidirectional relationship where the commensal feeds on host nutrients

May modify surroundings to make them favourable for other microorganisms

what is an example of commensalism

microorganisms on skin

what are the types of harmful symbiotic relationships

predation

parasitism

amensalism

what is predation

Predator species attacking, often killing its prey

what is parasitism

One obtains nutrients/benefits from host who is harmed

when is parasitism succesful and unsuccesful

Successful if co-existence is maintained

Failure if host dies or immune response elicited

what is an example of parasitism

Rickettsia typhi in human from rats (murine typhus)

the tropical disease schistosomiasis

what is amensalism

Harmful effects of one organism on another, unidirectional

what are examples of amensalism

antibiotic, bacteriocin production

What is competition in symbiotic relationships

A limiting nutrient resource utilised by different organisms – intra- and inter-specific competition

What is the competitive exclusion principle (Gause, 1934)

The dominant organism occupies physical space, acquires nutrients, and outgrows and outnumbers competing populations, often leading to extinction or specialisation

why does the ability of microbes to cause disease vary between species

Different virulence factors, modes of entry, etc

What are the three components of the innate immune system

Physical barriers, Chemical barriers, Cellular responses

How does normal microbiota help the host resist pathogens

Particularly on the skin and in the gut, via competitive exclusion

What were the characteristics of co-diversified species

They had smaller genomes and greater sensitivity to temperature and oxygen

How were co-diversified species transmitted

Between people in proximity or down generations

What happened to species that did not co-diversify

They were more likely to survive outside the body and were transmitted through water and soil compared to non-human primates

What were the differences in the gut microbiota of humans compared to non-human primates

Lower diversity in gut, increased Bacteroides abundance, decreased Methanobrevibacter and Fibrobacter

What factors changed the human microbiome

Dietary changes (e.g., cooking), agriculture, population size/density

Are internal tissues free of microorganisms

yes

Are surface tissues readily colonised by microbial species

yes

Is microflora acquisition a random or selective process

selective

Is the microbial niche stable

It is dynamic but generally stable

What can alter the microbiota

Medication, ageing, disease

What does host survival depend on

A complex defence network to keep pathogens and foreign material out and prevent parasitism or pathogenicity

What is 16S ribosomal RNA sequencing

A method to identify and compare bacteria and archaea

Why do all bacteria and archaea have 16S rRNA

It is a conserved gene present in all bacteria and archaea

How much phylogenetic information does 16S rRNA contain

Enough phylogenetic information (1500 bp)

What are highly conserved regions used fo

Primer design

What are highly variable regions used for

They have adequate variation to infer phylogenetic relationships and compare species

When does the development of a lifelong symbiotic relationship with microbes begin

at birth

What is the microbial community of the birth canal rich in

Lactobacill

What does breast milk do

It reinforces development and selects for symbiotic species

What are the effects of microbiota on human health

Influence metabolic function

protect against opportunistic infection

modify gene expression

immune system communication

produce neurotransmitters for neurological function and behaviour

What is the skin's role as a barrier

It is a mechanically strong barrier to microbial invasion with thick, tightly packed keratinocytes and continuous outer epithelial cell shedding

Is the skin microbiota shaped by physiological characteristics

yes

Do resident organisms normally grow on/in the skin

Yes. Their presence is fixed in well-defined distribution patterns

What are transient microorganisms

Temporarily present microorganisms on the skin

Does the mode of delivery affect initial colonisation

Yes. Initial colonisation is dependent on delivery mode

What are the long-term effects of delivery mode on health

unknown

What are the microbial differences associated with different delivery modes

Greater abundances of Firmicutes (Streptococcaceae spp.), Bacteroidetes and Proteobacteria (betaproteobacteria and gammaproteobacteria )

What happens to the skin microbiota during puberty

It is restructured. Increased hormones stimulate sebum production, favouring lipophilic organisms such as Propionibacterium spp., Corynebacterium spp., and fungal Malassezia spp

What does P. acnes produce and what effect does it have

P. acnes produces coproporphyrin III, which promotes S. aureus aggregation and biofilm formation

What is common acne, and what is it caused by

A chronic skin inflammatory condition. P. acnes is the most abundant organism, with distinct gene expression profiles between people with and without acne

what contributes to common acne

Genetics, immune/barrier defects, and microbiome-environment interactions

What is atopic dermatitis (eczema)

A chronic relapsing inflammatory disease involving epidermal barrier impairment, immune cell activation, and altered microbial community

Is it known whether changes in atopic dermatitis are cause or consequence

no

What is the role of S. aureus in eczema

is a poorly understood driver of eczema

Strain-dependent ability to trigger cutaneous immune response

δ-toxin induces mast cell degranulation

α-toxin induces monocytic IL-1β production (++ IL-17 from T-cells)

what is body odour produced by

Apocrine sweat glands – odourless secretion

Malodourous microbial metabolites

what do Corynebacterium spp. produce

volatile fatty acids › Intense cheese-like odour

What are the microbiota of the nasal cavity and nasopharynx (upper respiratory tract)

S. aureus, S. epidermidis (may be pathogenic when translocated)

S. pneumoniae (also an otopathogen with H. influenzae, M. catarrhalis)

How do these bacteria influence lower respiratory tract infections

hey influence the severity of LRTI caused by viruses by modulating the host immune response

What are the microbiota of the oropharynx

α-haemolytic streptococci – S. oralis, S. milleri, S. gordonii, S. salivarius

What are the microbiota of the external ear

they resemble that of the skin – coagulase-negative staphylococci, Corynebacteria, fungi

What is the microbiota of the middle ear

There is no scientific consensus on the "healthy" microbiota of the middle ear; it has low biomass

What is otitis media

Middle ear disease – an ear infection common in children

how are bacteria thought to reach the middle ear

They translocate via the pharyngotympanic (Eustachian) tube

Do acute cases of otitis media often resolve on their own

yes

What is chronic suppurative otitis media

A condition where the eardrum bursts; perforations heal on their own in most cases

Is there a resident microbiota in the lower respiratory tract

It is unknown if they exist as resident or transient populations

What genera are found in the lower respiratory tract

Pseudomonas, Streptococcus, Prevotella, Fusobacterium, Veillonella

How are sparse bacterial populations normally removed

By mucus from goblet cells, alveolar macrophages, and bactericidal lysozyme

What pathogens cause pneumonia

S. pneumoniae, H. influenzae, Mycoplasma pneumoniae

What happens in pneumonia

Inflammation and accumulation of fluids and leukocytes in the alveoli; can lead to pleurisy, making breathing painful

What viruses cause pneumonia

Influenza, viral pneumonia, SARS, MERS

What viral causes of pneumonia also cause skin rashes

Measles, rubella, chickenpox/shingles

Are fungal infections of the respiratory tract common

Symptomatic infections are rare in healthy individuals

What structures of the eye interface with the environment

Cornea, conjunctiva, tear film, eyelids

What is the dominant genus in the eye microbiota

Staphylococci (coagulase-negative commensals)

Is the "healthy" genera composition of the eye well-established

No. It is debated

Is the eye an immunologically privileged organ

yes

Why is the eye considered immunologically privileged

It is highly vascularised but requires sophisticated, tight regulation to avoid intraocular inflammation

What inflammatory diseases are associated with non-ocular microorganisms

Infectious uveitis caused by Herpes simplex virus, Toxoplasma gondii, Varicella-zoster virus

How do organisms resist mechanical removal in the oral cavity

By adherence (flush into stomach and epithelial cell shedding)

What genera initially colonise the oral cavity

Streptococcus, Neisseria, Actinomyces, Veillonella, Lactobacillus

What happens when teeth first erupt

Anaerobes dominate due to anoxic conditions between teeth and gums – Porphyromonas, Prevotella, Fusobacterium

What do S. parasanguis and S. mutans do

They adhere to enamel

What does S. salivarius do

It adheres to buccal and gingival epithelia and colonises saliva

What is the eventual outcome of oral biofilm formation

Dental plaque, caries, gingivitis, periodontal disease