medical microbiology i - lecture7

MEDICAL MICROBIOLOGY I

Lecture 7Lecture 7

Normal Flora in Health and Disease

Normal Flora

In a healthy human, the internal tissues (e.g.

brain, blood, CSF, muscles) are normally free

of microorganisms

The surface tissues (e.g. skin and mucosa The surface tissues (e.g. skin and mucosa

membranes) are constantly in contact with

environmental microorganisms and become

readily colonised by various microbial species

Normal Flora

The mixture of microorganisms regularly

found at any anatomical site is referred to as

the normal microbiota (the indigenous

microbial population, the microflora, or the microbial population, the microflora, or the

normal flora)

Bacteria make up most of the normal

microbiota, they are emphasised over the

fungi (mainly yeast) and protists

Normal Flora

Reasons to acquire knowledge of normal human microbiota:

An understanding of the different microorganisms at particular locations provides greater insight into the possible infections that might result from injury the possible infections that might result from injury to these body sites

A knowledge of the normal microbiota helps the physician-investigator understand the causes and consequences of colonisation and growth by microorganisms normally absent at a specific body site

Normal Flora

An increased awareness of the role that these

normal microbiota play in stimulating the host

immune response can be gained.

This awareness is important because the immune

system provides protection against potential

pathogens

Normal Flora

Three of the most important types of

symbiotic relationships are commensalism,

mutualism, and parasitism

Within each category the association may be Within each category the association may be

either ectosymbiotic or endosymbiotic

Normal Flora

One of the most important functions of our

normal flora is to protect us from highly

pathogenic organisms.

For example, in a normal (bacterially inhabited For example, in a normal (bacterially inhabited

animal), about 106 Salmonella must be

ingested in order to cause disease.

Normal Flora

However, when an animal has been

maintained in a sterile environment all of its

life (a gnotobiotic animal), the same level of

disease can be produced by as few as 10 disease can be produced by as few as 10

Salmonella.

This dramatic difference is simply due to

competition.

Normal Flora

The normal commensal population of microbes

participates in the metabolism of food

products, provides essential growth factors,

protects against infections with highly virulent protects against infections with highly virulent

microorganisms, and stimulates the immune

response.

The human fetus lives in a protected, sterile

environment,

A newborn is exposed to microbes from others

and environment

Normal Flora

The infants skin is colonised first, followed by

the oropharynx, GI tract, and other mucosal

surfaces

Throughout the life of an individual, this Throughout the life of an individual, this

microbial population continues to change

Changes in health can drastically disrupt the

delicate balance that is maintained among the

heterogeneous organisms coexisting within us

Normal Flora

Exposure of an individual to an organism can

lead to one of three outcomes

The organism can:

Transiently colonise the person Transiently colonise the person

Permanently colonise the person

Produce disease

Disease Formation

The process can result from microbial factors:

e.g. damage to organs caused by the proliferation

of the microbe or the production of toxins or

cytotoxic enzymes) cytotoxic enzymes)

or the hosts immune response to the organism

Strict pathogens (organism always associated

with diseases) - Mycobacteria tuberculosis,

Neisseria gonorrhea, Plasmodium sp.

Disease Formation

Opportunistic pathogens (organisms that are

typically members of the patients normal

flora) - Staphylococcus aureus, Escherichia coli,

Candida albicansCandida albicans

These organisms do not produce disease in

their normal setting but establish disease

when they are introduced into unprotected,

new sites

Skin

The adult human is covered with

approximately 2 m2 of skin

Commensal microorganisms living on or in the

skin can be either resident (normal) or skin can be either resident (normal) or

transient microbiota

Resident organisms normally grow on or in the

skin

Their presence becomes fixed in well-defined

distribution patterns

Skin

Those that are temporarily present are

transient microorganisms

They usually do not become firmly entrenched

and are unable to multiplyand are unable to multiply

Few microorganisms can penetrate the skin

because its outer layer consists of thick, closely

packed cells called keratinocytes

Continuous shedding of the outer epithelial

cells removes many of those microorganisms

adhering to the skin surface

Skin

The skin surface is not a favourable

environment for microbial colonisation - acidic

pH, high concentration of NaCl, lack of

moisture, and has certain inhibitory substances

Sweat glands release lysozyme (muramidase)

and cathelicidin (antimicrobial peptides)

Oil glands secrete complex lipids that may be

partially degraded by the enzymes from certain

Gram positive bacteria (e.g. Propionibacterium

acnes)

Skin

These bacteria can change the secreted lipids

to unsaturated fatty acids which as oleic acid

that have strong antimicrobial activity against

Gram negative bacteria and some fungiGram negative bacteria and some fungi

Some fatty acids are volatile and may be

associated with a strong odour

Most skin bacteria are found on superficial

cells, colonising dead cells, or closely

associated with the oil and sweat glands

Skin

Secretions from oil and sweat glands provide

the water, amino acids, urea, electrolytes, and

specific fatty acids that serve as nutrients

primarily for Staphylococcus epidermidis and primarily for Staphylococcus epidermidis and

aerobic corynebacteria

Gram negative bacteria generally are found in

the more moist region

The yeasts Pityrosporum ovale and P.

orbiculare normally occur on the scalp

Nose and Nasopharynx

The normal microbiota of the nose is found

just inside the nostrils

S. aureus and S. epidermidis are predominant

bacteria present and are found in bacteria present and are found in

approximately the same numbers as on the

skin of the surface

Nose and Nasopharynx

The nasopharynx, that part of the pharynx

lying above the level of the soft palate, may

contain small numbers of potentially

pathogenic bacteria such as Streptococcus

pneumoniae, Neisseria meningitidis, and

Haemophilus influenza

Diphtheroids, a large group of non-pathogenic

Gram positive bacteria that resemble

Corynebacterium, are commonly found in

both the nose and nasopharynx

Eye

At birth and throughout human life, a small

number of bacterial commensals are found on

the conjunctiva of the eye

The predominant bacterium is S. epidermis The predominant bacterium is S. epidermis

followed by S. aureus, Haemophilus sp. and S.

pneumoniae

External Ear

The normal microbiota of the external ear

resemble those of the skin, with coagulase-

negative staphylococci and Corynebacterium

predominatingpredominating

Fungi: Aspergillus, Alternaria, Penicillium,

Candida, and Saccharomyces

Mouth

The normal microbiota of the mouth or oral

cavity contains organisms that resist mechanical

removal by adhering to surfaces like the gums

and teeth

The continuous desquamation (shedding) of The continuous desquamation (shedding) of

epithelial cells also removes microorganisms

Those microorganisms able to colonise the

mouth find a very comfortable environment due

to the availability of water and nutrients, the

suitability of pH and temperature, and the

presence of many other growth factors

Mouth

The oral cavity is colonised by microorganisms

from the surrounding environment within

hours after a human is born

Most microorganisms that invade the oral Most microorganisms that invade the oral

cavity initially are aerobes and obligate

anaerobes (Porphyromonas, Prevotella, and

Fusobacterium) become dominant due to the

anoxic nature of the space between the teeth

and gums

Mouth

As the teeth grow, Streptococcus parasanguis

and Streptococcus mutans attach to their

enamel surfaces

Streptococcus salivarius attaches to the buccal Streptococcus salivarius attaches to the buccal

and gingival epithelial surfaces and colonises

the saliva

The presence of these bacteria contributes to

the eventual formation of dental plaque,

caries, gingivitis, and periodontal diseases

Stomach

Owing to the very acidic pH (2 - 3) of the

gastric contents, most microorganisms are

killed

Less than 10 viable bacteria / mL of gastric Less than 10 viable bacteria / mL of gastric

fluid

Streptococcus, Staphylococcus, Lactobacillus,

Peptostreptococcus, and yeasts such as

Candida sp.

Small Intestine The duodenum contains few microorganisms

because of the combined influence of the

stomachs acidic juices and the inhibitory action

of the bile and pancreatic secretions

Gram positive cocci and rods comprise most of Gram positive cocci and rods comprise most of

the microbiota

Jejunum - Enterococcus faecalis, lactobacilli,

diphtheroids, and Candida albicans

Ileum - pH becomes more alkaline; anaerobic

Gram negative bacteria and members of the

family Enterobacteriaceae become established

Large Intestine

Has the largest microbial community in the

body (>400 different species; many exist in

large number)

The microbiota consist primarily of anaerobic, The microbiota consist primarily of anaerobic,

Gram negative bacteria and Gram positive,

spore-forming, and non-sporing rods

Several studies have shown that the ration of

anaerobic to facultative anaerobic bacteria is

approximately 300 to 1

Large Intestine

Yeast (Candida albicans) and certain protozoa

may occur as harmless commensals

Trichomonas hominis, Entamoeba hartmanni,

Endolimax nana, and Iodomoeba butschlii are Endolimax nana, and Iodomoeba butschlii are

common inhabitants

Bacteriodes thetaiontaomicron suits for

survival in the gut, where it is able to degrade

complex dietary polysaccharides

Large Intestine

Methanogenic bacteria are thought to remove

the products of fermentation by converting H2and CO2 to methane

Various physiological processes move the Various physiological processes move the

microbiota through the colon so an adult

eliminates about 3 x 10 microorganisms daily -

peristalsis and desquamation of the surface

epithelial cells

Large Intestine

To maintain homeostasis of the microbiota,

the body must continually replace lost

microorganisms

The bacterial population in the human colon The bacterial population in the human colon

usually doubles once or twice a day

Under normal conditions the resident

microbial community is self-regulating

Large Intestine

Competition and mutualism between different

microorganisms and between the

microorganisms and their host serve to

maintain a status quomaintain a status quo

If the intestinal environment change, the

normal microbiota may change greatly

Disruptive factors: stress, altitude change,

starvation, parasitic organisms, diarrhoea, and use

of antibiotics or probiotics

Genitourinary Tract

The upper genitourinary tract (kidneys,

ureters, and urinary bladder) is usually free of

microorganisms

In both female and male, a few bacteria (S. In both female and male, a few bacteria (S.

epidermidis, Enterococcus faecalis, and

Corynebacterium sp.) usually are present in

the distal portion of the urethra

Genitourinary Tract

The adult female genital tract because of its

large surface area and mucous secretions, has

a complex microbiota that constantly changes

the females menstrual cyclethe females menstrual cycle

The major microorganisms are the acid-

tolerant lactobacilli (Lactobacillus acidophilus

or Dderleins bacillus) vaginal pH: 4.4. -

4.6

The Relationship between Normal

Microbiota and the Host

After a microorganism enters or contacts a

host, a positive mutually beneficial

relationship occurs that becomes integral to

the health of the hostthe health of the host

These microorganism become the normal

microbiota

The microorganism produces or induces

deleterious effects on the host; the end result

may be disease or even death of the host



Some microorganisms are pathogens

They are prevented from causing disease by

competition provided by the normal

microbiotamicrobiota

The normal microbiota use space, resources,

and nutrients needed by pathogens

They may produce chemicals that repel

invading pathogens



These microbiota prevent colonisation by

pathogens and possible disease through

bacterial interference

Products made by colonic bacteria (e.g. Products made by colonic bacteria (e.g.

Vitamin B and K) also benefit the host

Studies using germ-free animals suggest a

strong correlation between the establishment

of stable microbial flora and the induction of

immune competency



The induction of normal faecal flora to germ-

free rodents stimulates the production and

secretion of angiogenin-4, an antimicrobial

peptide of intestinal Paneth cellspeptide of intestinal Paneth cells

The reconstitution of germ-free rodents with

flora from conventially raised siblings causes

the abnormal gut-associated lymphoid tissue

and intestinal lamina propria to resemble that

of the conventional animals



Although normal microbiota offer some

protection from invading pathogens, they may

themselves become pathogenic and produce

disease under certain circumstances, and then disease under certain circumstances, and then

are term opportunistic microorganisms or

pathogens

The opportunistic microorganisms are

adapted to the non-invasive mode of life

defined by the limitations of the environment

in which they are living



For example, streptococci of the viridans

group are the most common resident bacteria

of the mouth and oropharynx

If they are introduced into the bloodstream in If they are introduced into the bloodstream in

large numbers, they may settle on deformed

or prosthetic heart valves and cause

endocarditis

Opportunistic microorganisms often cause

disease in compromised hosts



There are many causes of this condition

including malnutrition, alcoholism, cancer,

diabetes, leukaemia, another infectious

disease, trauma from surgery or an injury, an disease, trauma from surgery or an injury, an

altered normal microbiota from the prolonged

use of antibiotics, and immunosuppression by

various factors (e.g. drugs, viruses, hormones

and genetic deficiencies)



Normal microbiota are harmless and are often

beneficial in their normal location in the host

and in the absence of coincident

abnormalities.abnormalities.

However, they can produce disease if

introduced into foreign locations or

compromised hosts

Microbes and Infection

These microbes colonised the body, but they

remain in the outer surfaces without

penetrating into sterile tissues or fluids

When a microbe has penetrated the host When a microbe has penetrated the host

defenses, invaded sterile tissues, and

multiplied INFECTION!

Invasion by most microorganisms begins when

they adhere to cells in a persons body


Symptoms of disease are determined by the

function of tissue affected, although systemic

responses, produced by toxins, and host

defense responses may also occurdefense responses may also occur

The seriousness of the symptoms depends on

the importance of the organ affected and the

extend of the damage caused by the infection


The bacterial strain and inoculum size are also

major factors in determining whether disease

occurs; however, this can vary from a

relatively small inoculum to a very large relatively small inoculum to a very large

inoculum

Pathogenicity of Microbes and Virulence

Adherence is a very specific process, involving

lock-and-key connections between the

microorganism and cells in the body

Some microorganisms that invade the body Some microorganisms that invade the body

produce toxins

Some diseases are caused by toxins produced

by microorganisms outside the body


After multiplication begins, one of three

things can happen:

Microorganisms continue to multiply and

overwhelm the bodys defensesoverwhelm the bodys defenses

A state of balance is achieved, causing chronic

infection

The body - with or without medical treatment -

destroys and eliminates the invading

microorganism


Whether the microorganism remains near the

invasion site or spreads to other sites depends

on such factors as whether it produces toxins,

enzymes, or other substancesenzymes, or other substances

Pathogenic action:

Tissue destruction

Toxins

Endotoxin and cell wall components

Exotoxins

Immunopathogenesis


1. Tissue destruction

By products of bacterial growth, especially

fermentation result in the production of acids,

gas, and other substances that are toxic to tissuegas, and other substances that are toxic to tissue

Many bacteria release degradative enzymes to

breakdown the tissue, thereby providing food for

the growth of the organisms and promoting the

spread of the organisms, especially if blood

vessels are involved


2. Toxin

Bacterial components that directly harm tissue or

trigger destructive biological activities

Cause lysis of cells, and specific receptor-binding Cause lysis of cells, and specific receptor-binding

proteins that initiate toxic reactions in a specific

target tissue or initiate a systematic response (e.g.

fever) by promoting the inappropriate release of

cytokines


3. Endotoxin and other cell wall components

Peptidoglycan and its breakdown products, as well

as teichoic and lipoteichoic acids are released, and

these stimulate toxin-like pyrogenic acute-phase these stimulate toxin-like pyrogenic acute-phase

responses

Lipopolysaccharide produced by Gram -ve bacteria is

an even more powerful activator of acute-phase and

inflammatory reaction

Endotoxin binds to specific receptors on

macrophages, B cells, and other cells and stimulates

the production and release of acute-phase cytokines


Endotoxin also stimulates the growth (mitogenic) of B

cells

At low concentration, endotoxin stimulates the

mounting of protective responses such as fever,

vasodilation, and the activation of the immune and vasodilation, and the activation of the immune and

inflammatory responses

High concentration of endotoxin activate the

alternative pathway of complement and promote

high fever, hypotension, shock produced by

vasodilation and capillary leakage, and disseminated

IV coagulation stemming from the activation of the

blood coagulation pathways


4. Exotoxins

Proteins that can be produced by bacteria and

induce cytolytic enzymes and receptor-binding

proteins that alter the function or kill the cell

In many cases, the toxin gene is encoded on a In many cases, the toxin gene is encoded on a

plasmid or a lysogenic phage

Super-antigens are a special group of toxins

These molecules activate T cells by binding

simultaneously to a T-cell receptor - life threatening

autoimmune-like responses by stimulating the

release of large amounts of interleukins


5. Immunopathogenesis

The symptoms of a bacterial infection are

produced by excessive immune and inflammatory

responses triggered by the infectionresponses triggered by the infection

The acute-phase response to cell wall

components, especially endotoxin, is an initial

phase of the protective antibacterial response but

can also cause the life threatening symptoms

associated with sepsis and meningitis

Bacteria (Infection and Colonisation)

Infection: local or systemic infection

Local infection - limited to specific part of the

body where the microorganisms remain

Systemic infection - microorganisms spread and Systemic infection - microorganisms spread and

damage different part of the body

When a culture of the persons blood reveals

microorganisms BACTERAEMIA

When bacteraemia results in systemic

infection SEPTICAEMIA


Infection that appear suddenly or last a very

short time acute infection

Infection that occurs slowly, over a very long

period, and may last months or years period, and may last months or years

chronic infection


Infection in a wound is indicated by the

appearance of inflammation and pus

The patient may become pyrexial, and wound

swab will indicate the presence of large swab will indicate the presence of large

numbers of causative organisms


When colonisation occurs, several species of

bacteria may be present, often referred to as

mixed bacterial growth on laboratory reports

Colonisation is of clinical significance because Colonisation is of clinical significance because

the organism may multiply in large numbers to

form a reservoir

Colonisation is usually the precursor to infection

when outbreaks occurs, and even if the original

patient escapes the clinical signs and symptoms

of disease, cross-infection may still occur

Overview of Bacterial Pathogenesis

1. Maintain a reservoir. A reservoir is a place to live

and multiply before and after causing an infection

2. Initial transport to and entry into the host

3. Adhere to, colonise, and/or invade host cells or 3. Adhere to, colonise, and/or invade host cells or

tissues

4. Multiply or complete its life cycle on or in the

host or the hosts cells

5. Damage the host

6. Leave the host and return to the reservoir or

enter a new host

Gaining Access1. Inhalation

Through respiratory tract (nose or mouth), aerosols,

droplet transmission

The longer the period of exposure, the greater the

risk of inhalation

e.g. influenza virus e.g. influenza virus

2. Ingestion

Through mouth into gastrointestinal tract

e.g. food poisoning, typhoid fever

e.g. of causative agents: Salmonella, Shigella, E. coli,

Vibrio and the virus causing poliomyelitis enter by

being ingested

Bacterial Adherence Factors that Play a

Role in Infectious Diseases

Adherence Factor Description

Fimbriae Filamentous structures that help attach bacteria to other

bacteria or to solid surfaces

Glycocalyx or capsule A layer of exopolysaccharide fibers with a distinct outer

margin that surrounds many cells; it inhibit phagocytosis and

aids in adherence. When the layer is well organised and not aids in adherence. When the layer is well organised and not

easily washed off, it is called a capsule

Pili Filamentous structures that bind prokaryotes together for

the transfer of genetic material

S Layer The outermost regularly structured layer of cell envelopes of

some bacteria that may promote adherence to surfaces

Slime Layer A bacterial film that is less compact than a capsule and is

removed easily

Teichoic and

lipoteichoic acids

Cell wall components in Gram +ve bacteria that aid in

adhesion

Invasion of Host Tissues

Pathogens often actively penetrate the hosts

mucous membranes and epithelium after

attachment to the epithelial surface

This may be accomplished through production This may be accomplished through production

of lytic substances that alter host tissue by:

Attacking the extracellular matrix and basement

membranes of integuments and intestinal linings

Degrading carbohydrate-protein complexes

between cells or on the cell surface (glycocalyx)

Disrupting the cell surface

Microbial Defenses Against Host

Immunologic Clearance

1. Encapsulation

2. Antigenic mimicry

3. Antigenic masking

7. Inhibition of

chemotaxis

8. Inhibition of

phagocytosis4. Antigenic shift

5. Production of anti-

immunoglobulin

proteases

6. Destruction of

phagocytes

phagocytosis

9. Inhibition of

phagolysosome fusion

10. Resistance to

lysosomal enzymes

11. Intracellular

replication

Leaving the Host

The last determinant of a successful bacterial

pathogen is its ability to leave the host and

enter either a new host or a reservoir

Unless a successful escape occurs, the disease Unless a successful escape occurs, the disease

cycle will be interrupted and the

microorganism will not be perpetuated

Passive escape occurs when a pathogen or its

progeny leave the host in faeces, urine,

droplets, saliva, or desquamated cells

Questions

Define:

i) Normal flora

ii) Opportunistic organisms

iii) Pathogenic organismsiii) Pathogenic organisms

iv) Bacteraemia

v) Septicaemia

Questions

Name the normal flora on skin, respiratory

tract, digestive tract and urinary tract.tract, digestive tract and urinary tract.

Questions

What are the bacterial pathogenic actions?

medical microbiology i - lecture7

Documents

normal flora bacteria

diseasenormal flora

endosymbioticnormal

environmentnormal flora

normal microbiota play

usnormal flora exposure

protistsnormal flora

specific body sitenormal