Types of organisms that interact with host
Parasite
o
o
o
Relationship wherein one gets a benefit and the other is harmed
Good for one; bad for the other person
Aka parasitism
o
o
o
Both organisms derive a benefit
Two of them get advantage from each other
Aka mutualism
Sybiont
Commensal
o
o
o
o
o
Relationship wherin one organism benefits while the other organism is
neither benefited nor is harmed
Only one gets a benefited; other not benefit nor harmed
Normal Flora in our body are commensal
 We humans benefit from this bacteria; but bacteria don't derive
any benefit from us; they're also not harmed when they stay in
our body
Resident Flora
 Organism which is permanently situated in a particular site in
the body
 Stay there all the time
Transient Flora
 Organism which only stay in a particular body site for
temporary time only
 Normal bacteria
 Sometimes wash away
 They come and go
Exogenous Colonization by pathogens is blocked by bacerial interference
o Bacteria gives a benefits
 It blocks the colonization of pathogens in our body (bad
bacteria)
 If good bacteria present in our body, the bad bacteria
just can't invade that easily; so normal bacteria blocks
colonization of bad bacteria in our body
 4 mechanisms listed
 Produce chemicals like bacterocin; make environment
unhospitable
Beneficial Effects of normal flora
o Intestinal Flora
 Involved in the synthesis of vitamins
 Bacteria makes vitamins for us i.e. Vit K and other ones
o By defense against microbial pathogens
 Accomplished thru
 Examples on how normal bac fight bad bacteria
 Bifido bacteria lowers the pH to 5; so in the intestine, it
will inhibit the growth of bad bac in intestine
In vaginal flora, the lactobacilli in the female genito
system
 They produce lactic acid which will work against
the gonorrhea bacteria in the vagina
 Prolong oral antibiotic treatment alters the flora of the
GIT trac
 If pt takes antibiotic for too long (not needed), he
may think it's good for him; not good -- you're
killing your own normal bacteria in the intestine -alters the flora in our intestines - not good - if
you kill your own bacteria in intestines
 Candida -- causes diarrhea
 Fungi will be the ones to
predominate -- causing diarrhea
and skin infections
 Staph aureus - bad bacteria will produce
toxins because you've killed the good
bacteria in the intestines; bad bacteria
now unblocked - now bad for the patient
 Priming of the immune system - these
normal bacteria in us not essential for our
life; we can still live even without these
normal bacteria in our body; but without
normal flora, our bodies always sterile -we'll become more prone to effection -our immune system not used to any
infection (dirty environment) --- priming
of the immune system ---our host
resistance gets exercise from the presence
of bad bacteria
 Animals placed in sterile environment
soon after birth have little immunity --animal not used to nonsterile ejnvrio -has
no normal flora
Normally Sterile in health
 Person has no sickness
 Sterile fluids/tissues in our body if not sick
 CSF is sterile; it's the water which runs inside our brain
and spinal cord
 Blood supposed to be sterile too; but not all the
time
 Tissues
 Bladder
 Fallopian tube
 Middle ear -- sterile due to anatomy; hidden
inside in a closed area
 Places in our body with no bacteria; not even normal
flora at all; very clean (if you're healthy)
 Remember fluids and tissues

o
o
Normal Organisms (depends on what site)
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Not expected to memorize all the bacteria/virus normally
present at different sites
We have a list; be familiar with what's present in the particular
sites
Skin
 Listed in book
 E even the fungi- -normal flora in skin
 Staph coccus, streptococcus ---composition of skin for a
 Bacteria

 Staph coccus epidermidis - predominant skin
flora not in guide
Mouth
 Tongue
 Streptococcus viridans
 Gingival crevices
 Anaerobic flora; in gums, oxygen low --ana flora
will thrive
 Nasopharynx
 Transient carriage --stay there temporarily -streptococcus pneumonia, nisseria, meningitus --bacteria that cause meningo coccymea --these
bacteria is just normal bacteria in the nose (in
nasopharynx); if immune system gets down,
these bacteria will cause problems
 co
Respi tract
Conjunctival tract
Digestive
 Esophagus and stomach -- normal flora - not much
 Whenever we swallow or food goes down, the
normal flora goes down too
 Most found in colon
 Where all our waste material is deposited
 Majority of normal flora in l intestine composed of
 Anaerobic bacteria 96-99%
 Bacteroides fragiles- most predominant
organism In the colon
 Small intestines
 If infant breastfed, bifido bacterium predominant flora
 If art fed, lactobacillus acidophilus
 Flora affected by what we eat
 Genito urinary tract
 Vagina -- child bearing age - flora in vagina
depends on the age
 Due to hormonal factors
 If age, lactobacili, streptococci, yeast
Bacteria and Disease
o Koch's Postulate
 Set of hypothesis put out by Dr. Koch

Trying to establish the causal relationship btw organism and
disease
 Is this bacteria the one that causes the disease in this
pt? why he died?
 Conditions set (4)
 Organism must be present in all patients with
that kind of disease
 Organism must be able to be cultured from the pt
 If you transfer that org to anoterh pt, the other
pt should also have same sickness as the other
person you got it from
Definition of Terms
o Disease - undesirable host parasite relationship ---interruption in
normal function
 In the context of organism causing sickness in a patient
o Infection
 Invasion of body by the bad bacteria or organism
o Commensal
 Already defined
 Organism which colonize body surfaces without doing harm
o Pathogens
 Organism which cause the disease (cause damage)
 Bad bacteria or bad virus
o pathogenicity -- ability to produce the disease
 Pathogenic bacteria
 Nonpathogenic
o Virulence
 Degree of pathogenicity
 Also defined as the quantitative measure of the degree of
pathogenicity
 Measured by LD50 aka lethal dose 50; another way of
measuring virulence
 LD 50 is the minimum conc. Of bacteria needed
to kill at least 50% of experimental animals to
determine the lethal dose of bacteria needed to
measure virulence
 Virulence is more detailed than pathogenicity
 2 bacteria example
 These bacteria has a capsule (protects it from
phagocytosis
 Other one doesn't have capsule
 Both bacteria are pathogenic --both can produce disease,
but which of these two is more virulent??
 Naturally, it's the one with capsule because our
body won't be able to get rid of this bacteria
(with the capsule)
 Capsule is a virulence factor
Factors Influence which will affect presence/absence of infection
o Portal of entry - where did the bacteria enter?
 How did it get to us?
 Thru mouth? Skin? Std (sexual transmitted), parenteral (blood
vessels)

o
o
Certain bac can only produce infection if they go thru correct
port
 Salmonella --if you rub this bacteria thru intact skin, you
won't develop typhoid fever --not correct port of entry
 Correct -- thru the mouth
 Dirty food --how you get typhoid fever
Virulence of the organisms
 Depends which org has which virulence factors which allow it to
invade/infect pt
Number of microbes
 The higher the conc of bacteria presentn, better chance to
cause infection
 Not an absolute concept; not always the case
 Example
 If you will compare shigella bacteria and salmonella
bacteria,
 Shigella bacteria -- even a low concentration; if
you let pt swallow a low concentration of this
bacteria, he will get sick of shigelosis --even if
low
 Salmonella - even if given high dose, pt willget
sick;
 So which organism is more virulent?
 Shigella -- the one with the low dose
o
Defensive powers of the host
 Host immunity
 Depends on our body resistance against infection if you get sick
or not
o
Microbes cause disease in 3 ways
 Mechanical
 Thru invasion of tissues/surfaces
 Bacteria have structures which will help it invade our
body
 Chemical
 Toxin production
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Microbiological
 Interaction of host immune mechanism
 Immunity - war btw our body's defense and the bacteria
Ex
 Surface molecules providing adherence --pili (fimbriae);
arm which helps it attach itself to mucosa of the patient
 Extracellular enzymes
 Collagenase, etc --- to melt obstacles for it to
infect our body
Difference btw the two types of Toxin by bacteria
o Exotoxin
 Produced by both gram positive and gram negative bacteria
 Located extracellularly -- bacteria will secrete the toxin outside
of its own self

o
o
Exo --outside
Endotoxin
 Produced only by gram negative bacteria
 Closely bound to the cell wall so it's not released outside
 Toxin remains in cell wall
 So it's released if bacteria is ruptured or destroyed -when endotoxin released
Toxicity
 Great for exotoxin; even a very low concentration can be very
fatal already
 Endotoxin - toxicity not so much unless there is a massive or
large amt of endotoxin released- pt will not develop serious
disease
o
Tissue
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o
Endotoxin
 Non specific -- any organ affected by toxin

o
o
Affinity specific for exotoxin
Neurotoxin -- specific for CNS
Enterotoxin -- for GIT tract
Others
Targetted to specific organs
Composed of LPS --lipopolysaccharide; lipid a especially of the
lipopolysaccharide (studied later)
 Heat labile
 Endotoxin cannot be destroyed by heat; exo can
Antigenicity
 The ability to stimulate an immune response
 Exotoxin --high antigenicity; very potent stimulator of immune
response
 Endo --weak antigenicity; important because
 Exotoxin can be converted into a vaccine (toxoid); because of
the high antigenicity of exotoxin --ability tostim response
 Toxoid
 Inactivated endotoxin; used as a vaccine to protect us
 Tetanus toxoid; tetanus antitoxin --Course of infectious disease
 Several periods
 Incubation period --encountered with pathogen
 Time from when the pt first encountered the pathogen
up to the time he developed signs and symptoms
 Pt has no signs and symptoms yet
 Organism is still hiding; still waiting; developing itself to
get stronger to cause sickness
 Prodromal period
 Period of disease onset
 Have nonspecific signs and symptoms which appear in
the patient
 Examples
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If child is going to develop measles, he will have
a prodromal period --he will show red eyes,
runny nose, fever --not measles ---not specific
Invasion period
 Of maximum illness
 Signs and symptoms of pt all come out already
 Child will start coming out with rashes already; now you
know he has measles --before you didn't know
Fervence
 Fever starts going down
Convalense
 Pt already well
Infection
o
o
o
o
o
o
o
o
o
o
o
Local infection
 Limited to limited area
General
 Widespread
Exogenous
 From outside body
Endogenous
 Inside
Latent
Dormant
 Organism is hiding in the body waiting for right time to cause a
problem
Nosochomial infection
Single
Mixed
Acute
 Short period of time disease come out
Chronic
 Disease takes long time
Epidemic
o
Lots of people get sick in such a short period of time
 Dengue,
o
o
Disease constantly present in low numbers
Always there, but not so many people get sick
o
o
o
Disease from animals that's transmitted to humans
Rabies
Leptospirosis
Endemic
Zoonotic
Bacteremia
o
Bacteria in blood
Septisemia
o
o
Bacteria in blood but it's multiplying
So there's signs and symptoms
Viremia
o
Virus travelling in blood
o
Toxins travelling in blood
Toxemia
Case vs Contact
Carrier
o
o
o
Person harboring organism with no signs and symptoms of disease
And is transmitting the organims
Doesn't appear sick; doesn’t feel anything --transmitting virus --so all
others get sick
o
o
o
Non living object which can carry organisms
Door knob, towel, telephone
Can get bacteria from these objects
o
Any insect (fly, mosquito)
 Which transfers virus, --malaria
Fomite
Vector
Methods of transmission of disease
o Direct contact
o Indirect contact