Download 07_Path___vir_Fact_path_I_2014

Institute for Microbiology, Medical Faculty of Masaryk University
and St. Anna Faculty Hospital in Brno
Miroslav Votava
PATHOGENICITY AND
VIRULENCE
FACTORS OF PATHOGENICITY
AND VIRULENCE – I
The 7th + part of the 8th lecture for 2nd-year students
April 8, 2013
Resistance of biofilm towards
toxic substances – revision
MICROBES IN THE BIOFILM FORM ARE ALWAYS
MORE RESISTANT THAN IN THE PLANKTONIC
FORM
• Higher resistance applies also to disinfectants and
antibiotics
• Differences in sensitivity sometimes amount up to 3 orders
• General mechanism of the higher resistance is not known
• In each microbe-antimicrobial combination the mechanism
can be different
Possible causes of higher
resistance of biofilm – revision
1. More difficult penetration of toxic matter
through the biofilm
2. Character of environment in the biofilm is
altered
3. Also the microbial population in the biofilm is
altered
Biofilm and disease 1 – revision
Biofilm takes part in the pathogenesis of
1. chronic infections in general
2. infections of implanted devices
• the progress of these infections is slow
• they are without distinctive symptoms
• acute exacerbations occur occasionally
• the effect of antibiotic therapy is transitory only
• after stopping antibiotics infections recur (even if
bacteria grown from them appear sensitive in vitro)
Biofilm and disease 2 – revision
Chronic infections of natural bodily surfaces
dental caries (oral streptococci, mainly Streptococcus mutans)
periodontitis (Gram-negative oral anaerobes)
otitis media (Haemophilus influenzae)
osteomyelitis (Staphylococcus aureus)
cholecystitis and cholangoitis (enterobacteriae)
prostatitis (Escherichia coli)
subacute bacterial endocarditis (oral streptococci)
pneumonia in cystic fibrosis (Pseudomonas aeruginosa)
Biofilm and disease 3 – revision
Chronic infections of artificial surfaces
central venous catheters (coagul. neg. staphylococci, candidae)
prosthetic heart valves (Staph. aureus, Staph. epidermidis)
joint prostheses (Staphylococcus aureus, Staph. epidermidis)
surgical sutures (Staphylococcus aureus, Staph. epidermidis)
vascular grafts (Gram-positive cocci)
endotracheal tubes (various bacteria and yeasts)
intrauterine contraceptive devices (Actinomyces israelii)
urinary catheters (E. coli or others, mainly Gram-negative rods)
contact lenses (Pseudomonas aeruginosa, Gram-positive cocci)
Possibilities of affecting the
biofilm I – revision
Prevention of the biofilm development
Now: modifying the surface of biomaterials (change of
charge)
impregnation of biomaterials with antimicrobials
(antibiotics, antiseptics)
In future: interference with quorum-sensing signals
inhibition of extracellular matrix production
inhibition of highly resistant persistors development
Possibilities of affecting the
biofilm II – revision
Disrupting the already present biofilm
Now: high concentration of an antimicrobial – so-called
antibiotic plug in a venous catheter
combination of antimicrobials with different
mechanisms of action
disruption of extracellular matrix – e.g. with enzymes
(polysaccharide lyases)
In future: use of molecules causing the autodestruction of
biofilm
Detection of biofilm 1 – revision
Phenotypic methods
• staining of biofilm on the inner wall of a vessel
(test tube, well in microplate)
= Christensen method
universal for most microbes
• character of colonies on agar with Congo red
for staphylococci only
negative – colonies red, glossy
positive – colonies black, rough
Biofilm production on glass and
on hardened polystyrene
Biofilm ─
Biofilm +
PS
PS
S
S
Inoculum: 0.5 McFarland scale; culture: Sabouraud broth
with 8 % glucose, 48 hrs, 37 °C
PS = polystyrene, S = glass
Positive production of slime on
agar with Congo red
Black colonies of a biofilm-positive staphylococcus strain
Detection of biofilm 2 – revision
Genotypic methods
• e.g. proof of a gene set called icaoperon responsible for the production
of intercellular adhesin in
Staphylococcus epidermidis
●●●
Pathogenicity
Pathogenicity = ability of a microbe to be
harmful to health and to cause disease
× Infectiousness = ability to cause infection
Infection – broader term than disease
In the disease the symptoms of disease are
present (the infection is manifest)
But the infection may proceed without
symptoms (inapparent infection)
Apart from infections microbes can cause
food poisoning, as well
Ecological remark
Ecology = science on mutual relations among
organisms and relations between organisms
and their environment
Symbiosis = close association of two different
organisms
Three forms of symbiosis:
Mutualism – both partners benefit from the
association and are unable to survive without it
Commensalism – the association is beneficial for
one partner and indifferent to the other
Parasitism – the association benefits one partner
and harms the other (the host)
→ consequence = pathogenicity
Infection
The definition of infection is not easy
• Infection = situation when the etiological
agent of infection invades an organism
and multiplies in it; or it settles on bodily
surfaces and acts adversely there
• × Colonization = settlement of bodily
surface by a nonpathogenic microbe
(or by a pathogen that does not cause
pathological symptoms there)
History of infectious diseases
Physiological thesis of Hippocrates:
The disease (incl. the infectious one) =
consequence of certain inadequacy of organism
Microbial antithesis of Pasteur and Koch:
The cause of the infectious disease is a microbe
Ecological synthesis:
= synthesis of physiological thesis and microbial
antithesis – for the occurence of the infectious
disease 1. the microbe, 2. the host and 3. their
environment are responsible
Relationship between the
microbe and the host
The relationship is dynamic and influenced
by the environment:
microbe
host
environment
Illness is not a rule – peaceful coexistence is
usually better for the parasite
In spite of that the host tries to get rid of the
parasite – to destroy, remove or at least
to localize it
Pathogenicity
Pathogenicity = the ability to cause a disease
It depends on both microbial and host
species
Particular microbial species is pathogenic for
a specific host species only, for another
species it may be non-pathogenic
This host species is susceptible to the
relevant microbial species, to a different
microbial species it can be resistant
Primary and opportune pathogens
Primary (obligate) pathogens → cause disease even
in otherwise healthy individuals = chiefly agents
of classical infections (diphtheria, typhoid fever,
plague, gonorrhea, tetanus, influenza, morbilli
etc.)
Opportunistic (facultative) pathogens → cause
disease under certain conditions or at a certain
disposition only = usually members of normal
flora
• when they reach another site in the body
• or when the immunity of the individual is lowered
Natural and experimental
pathogenicity – examples
Microbes naturally pathogenic for man & animals:
Staph. aureus, Francisella tularensis, Clostridium
botulinum, rabies v., tick-borne encephalitis v.
Microbes pathogenic for animals experimentally:
Bacillus anthracis, Streptococcus pneumoniae,
Clostridium tetani – mouse
Mycobact. tuberculosis, rickettsiae – guinea pig
Treponema pallidum, herpes simplex v. – rabbit
Microbes pathogenic for man only:
Neisseria gonorrhoeae, mnohé viry (VZV, CMV aj.)
Microbes non-pathogenic for man:
Majority of soil and water microorganisms
Opportunistic pathogens – I
Typical opportunistic pathogen:
Escherichia coli
A part of normal colonic flora (but <1 % only)
Outside the large intestine = pathogen
• cystitis, pyelonephritis, urosepsis
• cholecystitis, peritonitis
• wound infections
At lowered immunity (newborns):
• meningitis
• diarrhea (EPEC – serotypes O55, O111)
Opportunistic pathogens – II
Another opportunistic pathogen:
Staphylococcus epidermidis
Part of normal skin and mucosal flora
Outside the skin and mucosae = pathogen
• wound infections (also surgical: sternum, eye)
• cystitis
At lowered immunity:
• above all blood stream infections in individuals
with i.v. catheters, infections of implants and
other devices
• sepsis in newborns and neutropenic individuals
Virulence
Virulence = degree (measure) of pathogenicity
Virulence = property of certain strain of the microbe
– a pathogenic species can incorporate highly
virulent strains as well as almost avirulent ones
Indicator of strain virulence: ability to kill
LD50 = 50% lethal dose (the amount of microbe that
is able to kill exactly ½ of experimental animals)
Increasing virulence: repeated passages of the
strain (be cautious with the strains from
dissection material)
Attenuation = artificial weakening of virulence
(attenuated strains serve for the preparation of
vaccines)
Attenuation – an example
BCG-vaccine against TBC (bacille Calmette-Guérin)
Original strain – Mycobacterium bovis – is less
pathogenic for man than Mycob. tuberculosis
The selected strain was „tormented“ 12 years on
potato with bile until it lost most of its virulence
(it is almost avirulent)
In a normal newborn BCG causes only a local
process in the site of injection or in a regional
lymph node
Very rarely in an immunodeficient newborn it can
cause the generalized infection
MICROBE
Species:
Strain:
obligately
pathogenic
virulent
Individual: sensitive
Species:
opportunistically
pathogenic
non-pathogenic
avirulent
nonspecifically unresponsive
or specifically immune
susceptible
resistant
HOST
FACTORS OF
PATHOGENICITY
AND VIRULENCE – I
Three elements of pathogenicity
and virulence
1. Transmissibility (communicability) =
ability to be transmitted between hosts
2. Invasiveness = ability to:
- enter the host
ability to
- multiply within
=
overcome
- spread within
the defence
3. Toxicity = ability to do harm to the host
Transmissibility – I
It depends on
• the way of transmission – especially on
- the way in which microbes leave the body
- the amount of excreted microbes
- the portal of entry into other host
• the microbe tenacity – the degree of resistance
to the external environment
• the minimum infectious dose – the number of
microbes required for the start of infection
• the behaviour of the host – the abuse of the
host‘s defensive reflexes for the transmission
Transmissibility – II
Way of elimination from the body: not only
respiratory secretions and diarrhoeic stool are
infectious but every biological substance, as well
Typhoid fever – Salm. Typhi is more dangerous
in kidney than in gallbladder (urine × stool)
Amount of eliminated microbes: 102 new infectious
virions per 1 infected respiratory epithelial cell →
109 virions in 1 ml of respiratory secretion
Portal of entry: in general, the infection penetrates
better through mucosae than through skin (e.g.
respiratory infection with tick-borne encephalitis
virus acquired in a laboratory is more dangerous
and usually ends fatally:
)
Transmissibility – III
Microbe tenacity (stability in environment)
Resistant: bacterial spores (Clostridium
tetani), protozoal cysts (Giardia lamblia),
helminth eggs (Taenia saginata)
Delicate microbes rely on:
- direct transmission (sexual contact in
particular – gonococci, treponemae)
- biological vectors (ticks, mosquitoes –
borreliae, arboviruses)
- transmission by water (leptospirae,
shigellae)
Transmissibility – IV
Infectious dose (ID)
high: V. cholerae, salmonellae – roughly 108 cells
(high ID: always in an immune individual!)
low: shigellae
102 cells
gonococci, Mycob. tuberculosis
101
Coxiella burnetii (Q fever)
100
(!)
Host behaviour
Abuse of defence reflexes such as cough,
sneezing, diarrhoea
Goal-directed change of behaviour: rats infected
with Toxoplasma gondii lose fear of cats (so as
the parasite can finish its life cycle in cat bowel)
Recommended reading material
Paul de Kruif: Microbe Hunters
Paul de Kruif: Men against Death
Axel Munthe: The Story of San Michele
Sinclair Lewis: Arrowsmith
André Maurois: La vie de Sir Alexander Fleming
Hans Zinsser: Rats, Lice, and History
Michael Crichton: Andromeda Strain
Albert Camus: Peste (= The Plague)
[email protected]
Thank you for your attention