Pathogens in Perspective Flashcards
What are environmental pathogens?
Microorganisms that normally spend substantial part of their lifecycle outside human hosts
BUT when introduced to humans cause disease with measurable frequency
What are some examples of Pathogenic Bacteria?
Staphylococcus aureus
Clostridium tetani
Yersinia pestis
Campylobacter jejuni
E.Coli O157:H7
Vibrio cholerae
Leptospira interrogans
Borrelia burgdorferi
What are some examples of classes of Protozoans?
Amoebae
= Entamoeba histolytics
Ciliates
= Balantidum coli
Flagellates
= Giardia lamblia
Apicomplexans
= Cryptosporidium parvum
= Plasmodium vivax
What are some examples of viral pathogens?
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
What are some examples of fungal pathogens?
Candida albicans
Asperigillus fumioatus + flavus
Cryptococcus neoformans
Histoplasma capsulatum
What is the resident flora ?
= 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
What is the Gut microflora? How does it change over time?
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)
What are some functions of intestinal bacteria?
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
What are some factors influencing intestinal flora?
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
How does the human microbiota change with age?
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)
What are the different classes of pathogen? How is the severity of an infection determined?
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)
What are the 5 modes of transmission?
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
What are some portals to entry?
= 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
What is the Infectious Dose (ID)?
= minmum number of microbes required for infection to proceed
= microbes with small IDs have greater virulence
= varies between genus, species and strain
How do microbes attach to the host?
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
How do microbes survive host defences?
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
How do microbes cause disease?
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
What are the 4 distinct stages of clinical infections?
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
What are the different patterns of infection?
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
What are infections that go unnoticed called?
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
What are some portals of exit?
respiratory (mucus, sputum, nasal drainage, saliva)
skin scales
fecal exit
urogenital tract
removal of blood
(can re-infect, back into environment)
How to describe the persistency of microbes?
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
What are some different types of living reservoirs?
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
How do animals act as reservoirs and sources?
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
What are Nosocomial Infections?
= 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)
How to measure the frequency of cases?
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
How to measure disease and scale?
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)
