Pathogens in Perspective

Question 1

What are environmental pathogens?

Answer 1

Microorganisms that normally spend substantial part of their lifecycle outside human hosts

BUT when introduced to humans cause disease with measurable frequency

Question 2

What are some examples of Pathogenic Bacteria?

Answer 2

Staphylococcus aureus
Clostridium tetani
Yersinia pestis
Campylobacter jejuni
E.Coli O157:H7
Vibrio cholerae
Leptospira interrogans
Borrelia burgdorferi

Question 3

What are some examples of classes of Protozoans?

Answer 3

Amoebae
= Entamoeba histolytics

Ciliates
= Balantidum coli

Flagellates
= Giardia lamblia

Apicomplexans
= Cryptosporidium parvum
= Plasmodium vivax

Question 4

What are some examples of viral pathogens?

Answer 4

Gastroenteritis causing:
= Rotavirus
= Adenovirus
= Norwalk

Non Gastroenteritis causing:
= Polio
= Enteroviruses 68-71
= Hepatitis A + E

Environmental zoonotic viruses
= Ebola
= West Nile Fever

+ SARS-CoV-2

Question 5

What are some examples of fungal pathogens?

Answer 5

Candida albicans

Asperigillus fumioatus + flavus

Cryptococcus neoformans

Histoplasma capsulatum

Question 6

What is the resident flora ?

Answer 6

= the ‘norm’

= includes bacteria, fungi, protozoa, viruses, helminthes and arthropods

= most bacteria in large intestine

= internal organs, tissues + fluids are microbe-free

= unique to an individual

= large metabolic potential (microbial gene pool very large)

Microbial antagonism
= bacterial flora benefit host by preventing overgrowth of harmful microbes

Question 7

What is the Gut microflora? How does it change over time?

Answer 7

Pre-birth = gut thought to be sterile

After birth = acquire commensal bacteria from environment
(breaking of foetal membranes, handling, feeding)

= commensal bacteria dominate
(changes can lead to disease e.g. IBD)

= density of bacteria depends on location
(e.g. less in stomach compared to intestine and colon)

= most diversity in colon
(mostly anaerobic, with aerobic less abundant)

Question 8

What are some functions of intestinal bacteria?

Answer 8

Protective functions
= pathogen displacement
= nutrient competition
= receptor competition
= production of anti-microbial factors

Structural functions
= barrier fortification
= induction of IgA
= apical tightening of tight junctions
= immune system development

Metabolic functions
= control IEC differentiation + proliferation
= metabolise dietary carcinogens
= synthesise vitamens
= ion absorption
= salvage of energy
= ferment non-digestible dietary residue and endogenous epithelial-derived mucus

Question 9

What are some factors influencing intestinal flora?

Answer 9

Delivery method at birth

Antibiotics

Oral contraceptives

Radiation / Chemotherapy

Corticosteroids

Sugar in diet

Bactericidal chemicals in drinking water

Pesticides in food

Alcohol

Heavy metals

H.pylori

Gastrointestinal pH

Stress

Question 10

How does the human microbiota change with age?

Answer 10

As age increases, gut microbial community significantly changes
(new strains, increase in diversity)

Process of early development of the microbiota is hight unstable and idiosyncratic (structural or behavioural)

Microbiota also differs among children from different countries

Vaginal vs C-section also makes a difference
(returns to normal after about 6 weeks)

Question 11

What are the different classes of pathogen? How is the severity of an infection determined?

Answer 11

True pathogens
= capable of causing disease in healthy persons with normal immune defences
= very few microbes are always pathogenic
= e.g. influenza, plague bacillus, malarial protozoan

Opportunistic pathogens
= cause disease when the host’s defences are compromised or when they grow in part of body not natural
= more microbes are potentially pathogenic
= e.g. Pseudomas sp, Candida albicans

Severity of disease depends on:
= virulence
(virulence factor = characteristic or structure that contributes to ability of microbe to cause disease)

Question 12

What are the 5 modes of transmission?

Answer 12

Person to person transmission
= sexually transmitted diseases
= respiratory infections

Foodborne transmission
= insufficient cooking
= poor sanitation

Waterborne transmission (environmental)
= drinking water / swimming
= faecal-oral route

Airbone transmission (environmental)
= aerosols
= wastewater treatment plants, land application

Vector-borne transmission (environmental)
= bite of an animal host

Question 13

What are some portals to entry?

Answer 13

= characteristic route a microbe follows to enter tissues of body

Skin
= nicks, abrasions, punctures, incisions

GI tract
= food, drink

Respiratory tract
= oral and nasal cavities

Urogenital tract
= sexual, displaced organs

Transplacental entry

Question 14

What is the Infectious Dose (ID)?

Answer 14

= minmum number of microbes required for infection to proceed

= microbes with small IDs have greater virulence

= varies between genus, species and strain

Question 15

How do microbes attach to the host?

Answer 15

Adhesion
= microbes gain stable foothold at portal of entry
= dependent on binding between specific molecules on host and pathogen

e.g. fimbriae, flagella, adhesive slimes or capsules, pili

Question 16

How do microbes survive host defences?

Answer 16

Initial response of host defences come from phagocytes

Antiphagocytic factors
= used to avoid this phagocytosis

e.g. slime layer or capsule makes phagocytosis difficult
e.g. Legionella pneumophilia has ability to survive intracellular phagocytosis
e.g. species of Staphylococcus / Streptococcus produce leukocidins = toxic to white blood cells

Question 17

How do microbes cause disease?

Answer 17

Virulence factors
= to invade and establish themselves in host
= determine degree of tissue damage - severity

e.g. Exoenzymes
= digest epithelial tissues and permit invasion of pathogens

e.g. Toxigenicity
= capacity to produce toxins at site of multiplication
= ENDOtoxins = lipid A of LPS of gram-negative bacteris
= EXOtoxins = proteins secreted by gram-positive and gram-negative bacteria

e.g. Antiphagocytic factors
= help kill or avoid phagocytes, remain an irritant to host defences

Question 18

What are the 4 distinct stages of clinical infections?

Answer 18

Infection period
= time from initial contact with infectious agent to appearance of first symptoms
= agent is multiplying but damage is insufficient to cause symptoms
(several hours to several years)

Prodromal stage
= vague feelings of discomfort, nonspecific complaints

Period of invasion
= multiplies at high levels
= becomes well established
= more specific signs and symptoms

Convalescent period
= as person begins to respond to the infection
= symptoms decline

Question 19

What are the different patterns of infection?

Answer 19

Localised
= microbes enter body and remain confined to a specific tissue

Systemic
= infection spreads to several sites and tissue fluids (usually in bloodstream)

Focal
= infectious agent breaks loose from local infection and is carried to other tissues

Mixed
= several microbes grow simultaneous at infection site (polymicrobial)

Primary
= initial infection

Secondary
= another infection by a different microbe

Acute
= comes on rapidly, with sever but short-lived effects

Chronic
= progresses and persists over a long period of time

Question 20

What are infections that go unnoticed called?

Answer 20

Asymptomatic (subclinical)
= although infected, host does not show signs of disease

= in-apparent infection = so person does not seek medical attention

e.g. Typhoid mary, covid-19

Question 21

What are some portals of exit?

Answer 21

respiratory (mucus, sputum, nasal drainage, saliva)

skin scales

fecal exit

urogenital tract

removal of blood

(can re-infect, back into environment)

Question 22

How to describe the persistency of microbes?

Answer 22

Apparent recovery of the host does NOT always mean the microbe has been removed

Latency
= after initial symptoms, microbe can periodically become active and produce a recurrent disease
= person may or may not shed it during latent stage
= e.g. herpes, chicken pox and shingles

Chronic carrier
= person with a latent infection who sheds the infectious agent

Reservoir
= primary habitat of pathogen in natural world
(human or animal carrier, soil, water, plants)

Source
= individual or object from which an infection is actually acquired

Question 23

What are some different types of living reservoirs?

Answer 23

Carrier
= individual who inconspicuously shelters a pathogen and spreads it to others
= may or may not have experienced disease

Asymptomatic carrier
= INCUBATION carriers = spread infectious agent during incubation perioid
= CONVALESCENT carriers = recuperating without symptoms
= CHRONIC carriers = individuals who shelter infectious agent for a long time

Passive carrier
= e.g. contamination healthcare provider picks up pathogens and transfers to other patients

Question 24

How do animals act as reservoirs and sources?

Answer 24

Vector
= animal that transmits an infectious agent from one host to another
= many are arthropods (fleas, mosquitoes, flies and ticks)
= some larger animals can also spread infection

Biological vectors
= actively participate in pathogen’s life cycle

Mechanical vector
= not necessary to life cycle of an infectious agent and merely transports it without being infected

Non Living reservoir
= soil, water and air

Zoonosis
= an infection indigenous to animals BUT naturally transmissible to humans
= at least 150 exist
= impossible to eradicate without eradicating the animal reservoir

Question 25

What are Nosocomial Infections?

Answer 25

= diseases that acquired or developed during a hospital stay

e.g. from surgical procedures, equipment, personnel, exposure to drug-resistant microorganisms

= most commonly involved urinary tract, respiratory tract, surgical incisions

= mostly involve gram-negative intestinal flora
(E. coli , Psuedomonas spp, Staphylococcus spp)

= >33% of these infections are preventable (2-4 million cases / year in US)

Question 26

How to measure the frequency of cases?

Answer 26

Prevalence
= total number of existing cases with respect to entire population
= usually represented by a percentage of population (n:100,000)

Incidence
= measures number of new cases over certain time period
= compared with the general healthy population
(n:100,000 / time period)

Mortality rate
= total number of deaths in population due to certain disease

Morbidity rate
= number of people afflicted with certain disease

Question 27

How to measure disease and scale?

Answer 27

Endemic
= disease that exhibits a relatively steady frequency over long period of time in a particular geographic locale

Sporadic
= when occasional cases are reported at irregular intervals

Epidemic
= when prevalence of disease is increasing beyond what is expected

Pandemic
= epidemic across continents

= measured by agencies (e.g. WHO, CDC, PHE)