Lecture -1 pathogenic bacteria Dr . Oruba Kuttof al- Bermani

Host Microbe- relationships

Microorganisms display a variety of complex relationships with other

microorganisms and with larger forms of life that serve as hosts for them.

A host is any organism that harbors another organism.

Symbiosis

Symbiosis is an association between two (or more) species. Meaning

“living together,” the term symbiosis encompasses a spectrum of

relationships. These include mutualism, commensalism, and parasitism.

mutualism

in which both members of the association living together benefit from the

relationshiplarge numbers of Escherichia coli live in the large intestine of

humans. These bac teria release useful products such as vitamin K, which

we use to make certain blood-clotting factors. Although the relationship

is not obligatory, E. coli does make a modest contribution toward

satisfying our need for vitamin K. The bacteria, in turn, get a favorable

environment in which to live and obtain nutrients 2`Many of the

bacteria on human skin are mutualistic. However, most of these

organisms are commensals, which indirectly benefit us by competing

with harmful organisms for nutrients and preventing those organisms

from finding a site to attach to and invade tissue

parasitism,

in which one organism, the parasite, benefits from the relationship,

whereas the other organism, the host, is harmed by itSome parasites

obtain com fortable living arrangements by causing only modest harm to

their host. Other parasites kill their hosts, thereby rendering themselves

homeless . The most successful parasites are those that maintain their

own life processes without severely damaging their hosts

commensalism

in which two species live together in a relationship such that one benefits

and the other one neither benefits nor is harmed. For example, many

microorganisms live on our skin surfaces and utilize metabolic products

secreted from pores in the skin. Because those products are released

whether or not they are used by microorganisms, the microorganisms

benefit, and ordinarily we are nei ther benefited nor harmed. The line

between commensalism and mutualism is not always clear. By taking up

space and utilizing nutrients, microbes that show mutualistic or

commensalistic behavior may prevent colonization of the skin by other,

potentially harmful, disease-causing microbes—a phenomenon known as

microbial competition. Hence these symbiotic relationships confer an

indirect benefit on the host. There is also a fine line between parasitism

and commensalism. In healthy hosts, many microbes of the large intestine

form harmless associations, simply feeding off digested food materials.

But a ‘harmless’ microbe could act as a parasite if it gains access to a part

of the body where it would not normally exist. The situation in which

both species harm each other without either benefiting is called

antagonism.

Contamination, Infection, and Disease

Contamination, infection, and disease can be viewed as a sequence of

conditions in which the severity of the effects microorganisms have on

their hosts increases. Contamination means that the microorganisms are

present. The surfaces of skin and mucous membranes can be

contaminated with a wide variety of microorganisms. Infection refers to

the multiplication of any parasitic organism within or on the host’s body.

If an infection disrupts the normal functioning of the host, disease occurs.

Disease is a disturbance in the state of health wherein the body cannot

carry out all its normal functions . Both infection and disease result from

interactions between parasites and their hosts. Sometimes an infection

produces no observable effect on the host even though organisms have

invaded tissues. More often an infection produces observable

disturbances in the host’s state of health; that is, disease occurs. When an

infection causes disease, the effects of the disease range from mild to

severe

Virulence refers to the intensity of the disease produced by pathogens,

and it varies among different microbial species. For example, Bacillus

cereus causes mild gastroenteritis, whereas the rabies virus causes

neurological damage that is nearly always fatal. Virulence also varies

among members of the same species of pathogen. For example,

organisms freshly discharged from an infected individual tend to be more

virulent than those from a carrier, who characteristically shows no signs

of disease. The virulence of a pathogen can increase by animal passage,

the rapid transfer of the pathogen through animals of a species susceptible

to infection by that pathogen. As one animal becomes diseased,

organisms released from that animal are passed to a healthy animal,

which then also gets sick. If this sequence is repeated two or three times,

each newly infected animal suffers a more serious case of the disease than

the one before it. Presumably the mi crobe becomes better able to damage

the host with each animal passage The virulence of a pathogen can be

decreased by attenuation, by repeated subculturing on laboratory media

Koch postulates.

The first direct demonstration of the role of bacteria in causing disease

came from the study of anthrax by the German physician Robert Koch

Four criteria that were established by Robert Koch to identify the

causative agent of a particular disease, these include:

1. the microorganism or other pathogen must be present in all cases of

the disease

2. the pathogen can be isolated from the diseased host and grown in

pure culture

3. the pathogen from the pure culture must cause the disease when

inoculated into a healthy, susceptible laboratory animal

4. the pathogen must be reisolated from the new host and shown to be

the same as the originally inoculated pathogen

many organism that do not meet the criteria of posulates have been shown

to cuase the diseases For examples, Mycobacterium leprae, the causative

agent of leprosy, cannot be isolated in pure culture. additionally to

Neisseria gonorrhoeae there is no animal model of infection even though

the bacteria can readily be cultured in vitro

Figure-1:- Experimental Koch postulates

The Disease Process

How Microbes Cause Disease

Microorganisms act in certain ways that allow them to cause disease.

These actions include gaining way access to the host, adhering to and

colonizing cell surfaces, invad ing tissues, and producing toxins and other

harmful meta bolic products. However, host defense mechanisms tend to

thwart the actions of microorganisms. The occur rence of a disease

depends on whether the pathogen or the host wins the battle; if it is a

draw, a chronic disease may result.

How bacteria cause disease

Bacterial pathogens often have special structures or phys iological

characteristics that improve the chances of suc cessful host invasion and

infection. Virulence factors are structural or physiological char

acteristics that help organisms cause infection and disease. These factors

include structures such as pili for adhesion to cells and tis sues, enzymes

that either help in evading host defenses or protect the organism from

host defenses, and toxins that can directly cause disease.

Direct action of bacteria

Bacteria can enter the body by penetrating the skin or mucous

membranes, by sexual transmission, by being ingested with food, by

being inhaled in aerosols, or by transmission on contaminated object

with an infectious agent. If the bacteria are immediately swept out of the

body in urine or feces or by coughing or sneezing, they cannot initiate an

infection. A critical point in the production of bacterial disease is the

organism’s adherence, or attachment, to a host cell’s surface. The

occurrence of certain infections depends in part on the interaction

between host plasma membranes and bacterial adhesions: are proteins or

glycoproteins found on attachment pili (fimbriae) and capsules . Most

adhesins that have been identified permit the pathogen to adhere only to

receptors on membranes of certain cells or tissues the capsules and

attachment pili are also antiphagocytic structures.

Attachment to a host cell surface is not enough to cause an infection. The

microbes must also be able to colonize the cell surface

. Colonization refers to the growth of microorganisms on epithelial

surfaces, such as skin or mucous membranes or other host tissues. For

colonization to occur after adherence, the pathogens must survive and

reproduce despite host defense mechanisms.

example, pathogenic bacteria on the surface of skin must withstand

environmental conditions and bacteriostatic skin secretions. The degree

of invasiveness of a pathogen—its ability to invade and grow in host

tissues—is related to the virulence factors the pathogen possesses and

determines the severity of disease ,most pathogen have additional

virulence factors like enzymes that enable the pathogen to invade tissues.

Among bacteria that release enzymes is Streptococci produce

hyaluronidase, or spreading factor. This enzyme digests hyaluronic acid,

a gluelike substance that helps hold the cells of certain tissues

together .Digestion of hyaluronic acid allows streptococci to pass

between epithelial cells and invade deeper tissues. Other example is

Staphylococcus aureus that produces coagulase to aid in infection

Coagulase is a two-edged sword: It keeps organisms from spreading but

also helps wall them off from immune de fenses that might otherwise

destroy them the bacterial enzyme streptokinase dissolves blood clots.

Pathogens trapped in blood clots free themselves to spread to other

tissues by secreting these virulence factors

A

B

Enzymatic virulence factors help bacteria invade tissues and evade

host defenses. (a) Hyaluronidase dissolves the “cement” that holds

together the cells that line the intestinal tract. Bacteria that produce

hyaluronidase can then invade deeper cells within the intestinal tissues.

(b) Coagulase triggers blood plasma clotting, allowing bacteria protection

from immune defenses. Streptokinase dissolves blood clots. Bacteria

trapped within a clot can free themselves and spread the infection by

producing streptokinase.

BACTERIAL TOXINS.

A toxin is any substance that is poisonous to other organisms. Some

bacteria produce toxins, which are synthesized inside bacterial cells and

are classified according to how they are released. Exotoxins are soluble

substances secreted into host tissues. Endotoxins are part of the cell wall

and are released into host tissues—sometimes in large quantities—from

Gram negative bacteria, often when the bacteria die or destroyed

endotoxins

are produced by certain Gram-negative bacteria. All endotoxins consist of

lipopolysaccharide (LPS) complexes They are relatively stable molecules

that do not display affinities

for particular tissues. Bacterial endotoxins have nonspecific effects such

as fever They also cause tissue damage in diseases such as typhoid fever

and epi demic meningitis

Exotoxins

are more powerful toxins produced by sev eral Gram-positive and a few

Gram-negative bacteria. Most are polypeptides, which are denatured by

heat, ultra violet light, and chemicals such as formaldehyde. Species of

Clostridium, Bacillus, Staphylococcus, Streptococcus, and several other

bacteria produce exotoxins

Some exotoxins are enzymes. Hemolysins were first discovered in

cultures of bacteria grown on blood agar plates. The action of these

exotoxins is to lyse (rupture) red blood cells

Virulence factors called leukocidins are exotoxins produced by many

bacteria, including the streptococci and staphylococci. These toxins

damage or destroy certain kinds of white blood cells called neutrophils

and macrophages

Table-1- types of bacterial toxinsb14.5 Properties of Toxins

property exotoxins endotoxinsOrganisms producing Almost all Gram-

positive; some Gram-negative

Almost all Gram-negative

Location in cell Extracellular, excreted into medium

Bound within bacterial cell wall; released upondeath of bacterium

Chemical nature Mostly polypeptides Lipopolysaccharide complex

Stability Unstable; denatured above 60°C and by ultravioletlight

Relatively stable; can withstand several hoursabove 60°C

Toxicity Among the most powerful toxins known (some are100 to 1 million times as strong as strychnine)

Weak, but can be fatal in relatively large doses

Effect on tissues Highly specific; some act as neurotoxins or cardiacmuscle toxins

Nonspecific; ache-all-over systemic effects orlocal site reactions

Fever production Little or no fever Rapid rise in temperature to high fever

Antigenicity Strong; stimulates antibody production andimmunity

Weak; recovery from disease often does notproduce immunity

Toxoid conversion and use

By treatment with heat or chemicals; toxoid used toimmunize against toxin

Cannot be converted to toxoid; cannot be usedto immunize

Many exotoxins have a special attraction for particular tissues. Like

Neurotoxins that act on tissues of the nervous system to prevent muscle

contraction (botulism) Enterotoxins, such as the toxin that causes cholera,

are exotoxins that act on tissues of the gut