Intro to Pathogens 9/9 Flashcards
Microbiota
Normal flora (bacteria, fungi, protozoa) that live on or within the bodies of animals and plants
- do not cause disease in healthy individuals
- have an adapted/noninvasice role by limitations of environment (commensalists or mutualists)
- extremely abundant in humans ~ 10^14
Can do good!
- Prevent or suppress pathogens
- Synthesize vitamins (Vitamin K), absorption of nutrients
- Abs produced to commensals cross-react with pathogens having related or shared Ag
- Bacteriocins
- Endotoxin release enhances immune response
Commensalism
Many of the normal flora neither hurt, harm, nor benefit the carrying host.
Mutualism
- Mutualisms are symbiotic interactions between two organisms in which both organisms benefit.
- One of the most important mutualistic roles of normal flora is as microbial antagonists.
- To prevent colonization by pathogens through bacterial interference
How does normal flora become pathogenic?
Can become pathogenic in in tissue(s) outside their normal niche
- Penicillinase + bacteria can interfere with therapy
- Confusion in Dxs due to resemblance to some pathogens
- Streptococcus viridans seeds bloodstream following dental procedure, settles on heart valve, infectious endocarditis
Resident and Transient Flora
- Resident: constant & well defined
–>Role of resident flora: Interference
- Competition for receptors or binding site on host cells
- Competition for nutrients
- Mutual inhibition by metabolic or toxic products
- Mutual inhibition by bacteriocins or antibiotics
- Transient: exposure to environment, does not cause disease or establish permanently
- Colonization: establishment of a microbial population
Acquisition of an new organism
Acquisition of normal flora
Environmental acquisition:
- In utero mammals are as sterile as any internal organ.
- Through contact with adults, older children, and the rest of their environment newborns acquire all their normal flora.
Large intestine: supports huge anaerobic populations, kept anoxic (O2 depleted) by facultative anaerobes.
- Birth canal, breast feeding, bottle feeding
- As the child grows facultative flora is acquired from oral exposure to feces.
- Strict anaerobes can colonize only after sufficient facultative organisms are established enough that oxygen concentrations are maintained at drastically reduced levels.
characteristics of a “pathogen”
Two mechanisms: invasion of tissue -or- production of toxins
- Invasiveness: ability to invade host tissues
- Capsules: some bacteria produce hyrdophilic gels that inhibit phagocytosis
- Adaptation: microenviconments of the host body provide habitats for bacteria that are capable of selective tissue invasion
- Extracellular enzymes: some bacteria produce enzymes like hyaluronidase or collagenase that degrade host tissues
Virulence:
the combination of invasiveness and toxigenicity
- LD = lethal dose
- LD50 = at what point 50% of the population dies - a measurement of virulence (%dead vs. dose)
- variability in virulence potential may be genotypic or phenotypic
Potential Pathogens
General factors –
Age, immunization history, prior illnesses or coexisting illness, trauma, nutrition, pregnancy, emotional state
Medical care:
- breaching of the skin (with intravenous devices or surgical incisions) or mucosal surfaces (with endotracheal tubes or bladder catheters)
- introduction of foreign bodies
- alteration of the natural flora with antibiotics, and treatment with immunosuppressive drugs.
Communicability
= transmission of disease
Communicability: infectious disease can be transmitted either directly (e.g. person to person) or indirectly (e.g. contaminated water).
Factors involved in the communicability of an infectious agent include:
- Source, including dormant or latent infections (carriers).
- Number of infectious agents released from a host.
- Capability of surviving transit from host to host.
- Percentage of the host population that is susceptible to the agent.
- ID50 (% infected vs. dose)
Toxigenicity (exotoxins and endotoxins)
- Toxigenicity: the production of toxins.
- Exotoxins: secreted proteins that are generally very toxic but heat labile. Exotoxins are found mostly in Gram-positive organisms.
- Endotoxins: complex polysaccharides (LPS) that are a part of the bacterial cell wall. These toxins are released when cells lyse, are generally heat stable, and found mostly in Gram-negative bacteria.
Stages of pathogenic Process
- Adhere
- Must evade local immune system
- Must replicate
- Must evade systemic immune system (need to downregulate adaptive response)
- Must escape body for transmission to new host
Generally:
- Intracellular pathogens generally produce chronic disease
- Extracellular pathogens generally produce acute disease
Stages of Infections
- Incubation period: replication phase – evading immune response
- Prodrome period: Non-specific symptoms
- Specific-illness period; Characteristic signs and symptoms
- Recovery period
- Latent infections
Viruses: characteristics and replication
- obligate intracellular parasites
- contain DNA or RNA, ss or ds
- surrounded by protein coat (“capsid”)
- certain viruses contain an additional phospholipid bilayer (“envelope”) surrounding capside that is derived from host cell
- Viral replication:
- virus recognizes and attaches to host (specific)
- viruses adsorb to host cell surface via receptor molecules (often temp. indepen)
- Penetration into host cell may occur in three ways (1. translocation of the PM, 2. pinocytosis into cytoplasmic vacuoles, 3. fusion of the PM with the viral envelope)
- non-enveloped viruses may enter via translocation or pinocytosis; enveloped viruses enter via fusion
- once inside, virus uncoats and rleases the viral genome for replication
- viral nucleic acid replication results in new viral protein that are packaged and released
Viruses: effects on host cells
Viruses can have one of several different effects on their cellular hosts.
Abortive infections
- result when a virus mistakenly infects a cell that does not permit viral replication
Cytolytic infections:
- lead to cell lysis and release of large numbers of virions
Persistent infections
- may be productive, latent or transforming
