paper no. : 09 animal cell biotechnology module :21 host

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Animal Cell biotechnology

Host Pathogen Interactions

Paper No. : 09 Animal Cell Biotechnology

Module :21 Host Pathogen Interactions

Principal Investigator: Dr Vibha Dhawan, Distinguished Fellow and Sr. Director

The Energy and Resources Institute (TERI), New Delhi

Paper Coordinator: Dr. Minakshi, Professor & Head, Lala Lajpat Rai University of Veterinary & Animal Sciences, Hisar

Content Writer: Dr. Hari Mohan, Assistant Professor, Maharshi Dayanand University, Rohtak

Paper Reviewer: Dr. Minakshi, Professor & Head, LalaLajpatRai University of Veterinary & Animal Sciences, Hisar

Co-Principal Investigator: Prof S K Jain, Professor, of Medical Biochemistry

Jamia Hamdard University, New Delhi

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Description of Module

Subject Name Biotechnology

Paper Name Animal Cell Biotechnology

Module Name/Title Host Pathogen Interactions

Module Id 21

Pre-requisites Basic knowledge of microbiology

Objectives 1. To understand defense mechanisms of host as well as the pathogen

2. Components of host-pathogen interactions

3. Basis of host-pathogen interactions

4. Types of interactions

5. Significance of host-pathogen interactions

Keywords Virulence factors, Commensalism, Symbiosis, Parasitism, Scavenging, Defense,

Endotoxin, Leucocidin, Hemolysin

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Table of Content:

1.) Introduction

2.) Koch’s Postulates: Experimental Host-Pathogen interaction

3.) Types of interaction

4.) Host defense mechanism

5.) Pathogen Defense

6.) Mechanism of Host-Pathogen Interaction

7.) Factors affecting pathogenicity

8.) Classification of Host Pathogen Interactions

9.) Basis for Host Pathogen Interactions

10.) Machine learning based Host-Pathogen interaction

11.) Summary and future scopes

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1. Introduction:

Every organic being is related, in the most essential yet often hidden manner, to that of all

other organic beings, with which it comes into competition for food or residence, or from

which it has to escape, or on which it preys… Charles Darwin

Every organism in this world is connected with each other for different purpose. This

connection or interaction is basically responsible for sustainable life on earth. Without these

interactions, ecosystem balance cannot be achieved on this earth. Organisms are surrounded

by millions of microbes, fungi, protozoa and viruses but few of them are able to cause disease

in the organisms. Pathogens invading a host need to interact with the host cells, multiply and

secrete virulence factors in order to cause disease. Currently, host-pathogen interaction has

become a rapidly emerging research area. Multidrug resistant microbes are posing serious

threat to humanity. Every year new kind of pathogens is haunting mankind. In order to

control and prevent these pathogens, basic understanding of their normal life cycle,

interaction with host cell and virulence factor will help in designing effective drugs and

vaccines. Occurrence of disease now can be simulated as fight between genes & proteins of

host versus pathogen and prevention is better than cure in this fight. For designing

appropriate preventive measures, host-pathogen interactions may play very important role.

Earlier, elucidation of host-pathogen interaction was based on experimental work (i.e. Koch

postulates) but with the development in computer sciences, in silico prediction of protein-

protein interaction between host and pathogen has become possible. It has made the

procedure of understanding host-pathogen interaction less cumbersome, less time consuming

and easy. However, software based host-pathogen prediction methods also suffer with the

drawback of less accuracy and it need to be rectified. Earlier studies of host-pathogen

interaction were carried out with respect to temperature, sunlight, season and population of

host/pathogen. However, with the use of model animals (C. elegans, Drosophila

melanogaster), host-pathogen interaction studies could be performed with ease as these

model animals have easy body plan, short life cycle and whole genome sequenced. These

model animals can mimic the replication mechanism of pathogens. Certain breakthrough

research such as secretion system of gram negative bacteria, virulence genes have given a

direction to the field of host-pathogen interaction. The future scenario of this research seems

promising in solving the mystery of newly emerging & multidrug resistant microbes by

designing rational therapeutic interventions and effective preventive strategies. This chapter

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will describe the different types of host-pathogen interaction, mechanism and latest advances

in this field.

2. Koch’s Postulates: Experimental Host-Pathogen interaction

These four postulates were the criteria for establishing relationship between microbe and

disease. These postulates were given by Robert Koch and Friedrich Loeffler in 1884 based on

the earlier concepts proposed by Jokob Henle and further refined by Robert Koch in 1890.

Initially these criteria were given for anthrax, cholera and tuberculosis but later on it was

generalized for most of the diseases. These are as following:

a. The etiological agent/microbe must be present in every case of the disease.

b. The agent must be isolated in pure culture from the diseased host.

c. When this pure culture microbe is inoculated in the susceptible host, the same disease

must be produced.

d. The same microbe must be isolated from the experimentally inoculated host.

These four postulates still hold true for most of the disease today also. But, Robert Koch

himself abandoned the universality of the first postulate when he came to know about the

asymptomatic carriers of cholera and typhoid. Currently asymptomatic carriers have been

reported in a number of diseases i.e. Poliovirus, Herpes simplex virus, Hepatitis C virus etc.

Second postulate doesn’t hold true for the organisms which cannot be isolated in pure culture

(such as certain virus, prions etc). All the susceptible hosts don’t exhibit the symptoms of

disease even exposed to the infectious agent. Therefore, in third postulate also, ‘must’ can be

replaced with ‘should’. With the advancement in technology, molecular Koch’s postulates

have been proposed which are as follow:

a. A signature nucleic acid sequence for a putative pathogen must be present in most

cases of a disease.

b. No or fewer copies of that signature nucleic acid sequence should be present in the

susceptible host without disease.

c. The copy number of pathogen associated gene should decrease or become non-

detectable when disease resolves.

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d. The copy number of pathogen associated gene should increase with the severity of

disease and it must be reproducible.

Molecular Koch’s postulates fulfill the criterion for most of the diseases. It only require

the signature sequence specific for a pathogen.

3. Types of interaction

Host and pathogen interacted in different ways for different purposes and every interaction

involves two or many organisms. Three basic types of host pathogen interactions are Neutral,

Positive and Negative interactions.

Neutral interactions- In these interactions neither the host nor the pathogen are benefitted or

suffered.

Positive interactions-In these interactions one or both the organisms are benefitted. Such

interactions involve Mutualism, Cooperation, Commensalism and Scavenging.

Negative interactions- In these interactions one of the organisms suffers. Such interactions

involve Predation, Competition, Amensalism and Parasitism.

3.1 Neutral Interactions

Interactions in which none of the two species are benefitted or harmed.

In few such interactions, no interactions are involved at all. Example: animals living

in the forest doesn’t necessary involve interactions with all the other animals.

While in others, interactions may be there because of the interdependency between

the organisms. Example: Plants provide food, oxygen as well as shelter to the animals

in exchange of the carbon dioxide and dispersal of seeds that animals do for them.

3.2 Positive Interactions

3.2.1 Mutualism: It is an obligatory relationship in which both the host and pathogen are

dependent on each other. During this interaction the organisms of one species grow, persist

and multiply at a higher rate in presence of their interacting species. Such interactions are

very common in insects as the food used by insects often lack essential vitamins and amino

acids. They gain those vitamins and amino acids from bacteria during symbiotic association

in exchange for the shelter and the nutrients.

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The aphid harbors the proteobacterium Buchnera aphidicola in its cytoplasm

which provides the insect with 10 essential amino acids. A mature insect has

millions of these bacteria in its body. When the insect is treated with

antibiotics, it dies.

Many marine invertebrates (sponges, jellyfish, sea anemones, corals, ciliates)

harbor endosymbiotic dinoflagellates called zooxanthellae within their tissue. The

hermatypic (reef-building) corals meet most of their energy requirements using

their zooxanthellae. In exchange for up to 95% of their photosynthate (fixed

carbon), the host provides zooxanthellae with the nitrogenous compounds,

phosphates, CO2 as well as protection from UV light.

3.2.2 Cooperation: It is a non-obligatory (unlike mutualism) but positive type of symbiosis

in which both the organisms are benefitted. In cooperation, the cooperating organisms when

kept away remain mortal even though they do not function properly.

In nematodes, including Eubostrichus parasitiferus, survive at the interface

between oxic and anoxic sulfide-containing marine sediments. Sulfides

containing marine sediments cover the animal in intricate patterns. The

bacteria help in decreasing the levels of toxic sulfide and also serve as food.

Two examples of a cooperative relationship include the association between

Desulfovibrio and Chromatium, in which the carbon and sulfur cycles are

related and the interaction of a nitrogen-fixing microbe with a cellulolytic

organism such as Cellulomonas. In the second example, the cellulolytic

organism releases the glucose from the cellulose which can be used by

nitrogen-fixing microbes.

3.2.3 Commensalism: It is a unidirectional process in which one is benefitted and the other

is neither benefitted nor harmed. The commensal is usually provided by the food and shelter

by the host. There is no direct dependency of the commensal on the host as the commensal

can survive even without the host. It is also said that this interaction is good for both in some

case. For example E. coli can infect immature immune systems of mammals and this action is

responsible for the production of antibodies in body also these microbes produce metabolites

that are necessary for host. Endogenous micro flora of humans also provides protection from

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other pathogen during whole life although these are also acquired by infection (Casadevall A

et al., 2000).

When the waste product of one organism is a substrate for the other organism.

Example: the process of Nitrification in which the ammonium ions are

oxidised to the nitrites by microorganisms like Nitrosomonas, and the nitrites

are then oxidised to nitrate by the Nitrobacter. Nitrobacter when associated

with the Nitrosomonas provides it with the nitrite that helps it to obtain

energy.

Commensalism occurs when one organism alters the environment to make it

suitable for the other. Example: Non-pathogenic E. coli lives in the gut of

human colon can grow well outside the colon also. When oxygen is present,

the facultative anaerobic E. coli can use it and even the obligatory anaerobic

organisms such as bacteroids can grow. Thus, anaerobes are benefitted but no

such benefits to the E. coli.

When a cattle feeds on a grass field, it disturbs the insects which are living in

grass field. This process helps cattle egrets to take their prey. It is a common

process showing commensalism because one is getting benefitted and other is

getting none or remains unaffected.

3.2.4 Scavenging: It is the type of interaction in which one organism called the scavenger

feeds on the dead bodies of the other. The other organism may be killed naturally or by other

animals. Scavenging cleans the environment by making the nutrients present in the dead

organism to re-enter the nutrient cycling process.

Example: hyenas, foxes and vultures are natural scavengers. Dogs, crows, ants

are the scavengers.

3.3 Negative Interactions

It is an interaction in which one organism is harmed by another or both are affected.

These are as follows-

3.3.1 Predation: In this type of interaction, one organism is benefitted while the other is

harmed. The one which attacks is known as the predator and the one being attacked is the

prey. Predation enhances the reproduction of the organisms that do the killing (predator) over

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the organisms being killed (prey). Predation also maintains the balance of different organisms

in nature by preventing the dominance of one species.

Some fungi can trap protozoa by their sticky hyphae, constricting or non-

constricting rings. Arthrobotrys traps the nematodes with its constricting rings,

obtains nutrition from its cytoplasm by growing the hyphae inside the prey.

Legionella is being ingested by ciliates which protects the bacteria from

chlorine. Thus offering the bacteria with protection as well as pathogenicity.

3.3.2 Competition: When organisms of different populations compete for the same

resources that are in limited supply which may be either for food or shelter. Competition can

be inter-specific or intra-specific. When competition occurs in between same species for

example in humans, it is known as intra-specific completion but when it occurs in between

different species for example humans and animals, it is known as inter-specific competition.

When one of the two organisms rules the environment by dominating the other one in terms

of shelter and nutrition, it will overshadow the other. Due to this one species can be

completely wiped out from earth. This is known as competitive exclusion principle.

Example: In chemostats, competition occurs among the different

microorganisms for the limited nutrients present. Finally at the end, one of the

populations dominates the rest.

3.3.3 Amensalism: It is an interaction in which actions of one organism have an adverse

effect on another. It occurs in between different species. It is a unidirectional interaction in

which the discharge of a specific compound by one organism has a negative effect on

another. It is a protective action for many organisms to protect it from predators.

Example: Antibiotics (Penicillin) produced by fungus can inhibit or kill the

microorganisms. Juglans nigra is a plant which secretes juglone to destroy

other small plants from its rooted area.

Algal blooms grow in water body and causes deaths of several fishes. In this

interaction algal bloom is not benefitted but the effect on fishes is negative.

Wild pigs disturbs upper layers of soil in search of food this will highlight

many insects that live under this layer is case of Amensalism relationship.

3.3.4 Parasitism: It is the interaction between two species in which the parasite is benefitted

at the expense of the host. They are different from predators as they do not necessarily kill

their host. In fact, it is beneficial for them to keep their host alive as the host provides them

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with food and shelter. Parasites can be classified either ecto-parasites or endo-parasites. But

in some cases, the effect of parasites is strong enough to cause diseases and finally the death

of the host. Parasites are divided into two types ect-parasites and endo-parasites. Ecto-

parasites remain on the surface of body for example blood sucking ticks. Endo-parasites lives

inside the body of host, for example plasmodium is an endoparasite for mosquito.

Example- Biorhiza pallida is wasp species responsible for gall formation in oak trees

and also known as oak apples. It lays eggs in buds of tree also injecting venom so that

bud can transform into gall and these eggs can take nutrients from that site.

Some parasites sucks blood from their host to get nutrients for example Culicoides

impunctatus takes blood from animals.

Brood Parasitism- It is an interaction in which one organism mimics egg pattern and

colour and this will help them to utilize parental care of host animal. For example-

cuckoo lays their eggs in crow’s nest to provide them parental care of crow.

4. Host defense mechanism

Living organisms have many defense mechanisms to avoid infection where body has direct

exposure to environment. For example- Eyes secrete lysozyme enzyme and tears that contain

salt. As eyes have direct contact with air that contain many pathogens so these secretions

avoid infections in eyes, secondly when we eat food that may have many pathogens cannot

survive in acidic content of stomach which is secreted by parietal cells. Approximately 2

liters of acid is secreted by these cells in stomach. The air we breathe may also have

pathogens that are restricted to the outer region of respiratory system because of mucous

secretion by goblet cells. These are the examples of various defense mechanisms of host

include:

Skin and mucosal secretions in host body resist pathogens from invading.

Non-specific local responses (example- pH, Enzymes, hormonal regulation)

Non-specific inflammatory responses due to immune system

Specific immune response (example: Lymphocytes, antibodies)

5. Pathogen Defense

When the pathogens invade the host, they are exposed to host immune system. In order to

invade and establish infection, they secrete many factors. In case of gram negative bacteria,

secretion systems I – VI have been found to be important in bacterial pathogenesis. Other

proteins such as toxins, iron scavenging factors, urease, catalase, adhesion molecules etc play

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crucial role in invasion and establishment of infection. Toxins are small molecules capable of

causing harm to the host on exposure by interacting with the biomolecules such as proteins or

enzymes. Important toxins identified are tetanus toxin, botulinum toxin, diphtheria toxin,

cholera toxin, LT toxin etc. These toxins have different mechanism of action and depending

on the concentration severity of the disease will appear. Effector proteins suppress the

immune system of host and thus help the pathogen in invading. Loss of effector protein T3SS

has been found associated with the conversion of Yersinia pestis to avirulent form. Enzymes

such as urease, catalase, oxidase etc are associated with pathogenesis. Urease has been found

to be associated with the pathogenicity of Mycobacterium. It is also important in

pathogenicity of Proteus mirabilis and Helicobacter pyroli. MAMs (multivalent adhesion

molecules) contribute significantly to microbial pathogenesis in high affinity binding with the

host cells. MAM7 found commonly on the surface of gram negative bacteria connect

pathogen to host cell via protein-lipid interaction and protein-protein interaction.

6. Mechanism of Host-Pathogen Interaction

Pathogen may invade the host via different routes and cause damage. Damage to the host may

happen in following ways:

A. Food borne intoxication- When a pathogen releases toxin after its introduction via

food causes serious effects on body functioning. For example- Clostridium botulinum.

It releases a neurotoxin that binds with high affinity receptors in cholinergic nerve

terminals and blocking the release of acetylcholine that is a neurotransmitter.

B. Colonization on host surface- Some pathogens release toxins and increase their

number to infect their host. Vibrio cholerae release toxin which has two subunits A

and B. B subunit makes pore in the enterocyte cell of intestine and A subunit has

enzymatic action which activates G protein which further activates CFTR (cystic

fibrosis transmembrane conductance regulator) that results in efflux of water and ions

from intestinal cell. An infected person releases as average of 20L of water which

make this disease life threatening.

C. Avoiding body’s barrier- Some pathogens has ability to break or cover themselves

from body immunity. This ability benefits them to increase their number and cause

disease. For example Mycobacterium tuberculosis multiplies in lung alveoli,

macrophage etc. when this pathogen enters it can protect themselves from

phagolysosome and macrophages undergo into necrosis. There are many pathways for

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entry like Fc receptors, mannose binding receptors, complement receptors etc. Some

pathways helps in avoiding the danger of host immunity. One such receptor is CR3

(complement receptor 3) receptor. This receptor helps in avoiding the fusion of

phagosome with lysosome if pathogen has host cholesterol on its surface (Pieters J

2008). Till now dendritic cells has represented the antigen to the regional lymph node

and activated T cells. But this pathogen has methods of protection from immunity of

host.

7. Factors affecting pathogenicity:

There are many factors which affect the capacity of the pathogen to cause disease. Same

microbe may not be able to cause disease in all the members of a population. These factors

includes:

Ability of a pathogen to adhere to a host surface- Pathogen can attach themselves

to the surface of host through specific binding between their ligand and receptor

present on the host cell.

Mutation rate in pathogen- Some pathogens are highly mutative; for example virus

can change their genome at high rate and this will change the entire virulence of virus

towards host. A host generates immunity based on the specific antigen that was used

by pathogen for invasion into host. If a pathogen is capable to change its genetic

makeup, it can avoid its host immunity barrier by changing that antigen.

Ability of a pathogen to colonize a host

Specific environment is required for colonization; for example Plasmodium

culicifacies is mosquito parasites that can colonies mosquito salivary gland, human

liver cells. This capability of pathogen is mandatory for its infection.

Ability of a pathogen to evade host’s defenses.

Any cut or injury on body can provide direct access to the body.

Number of pathogens.

8. Classification of Host Pathogen Interactions

This host-pathogen interaction procedure is classified into four stages.

a. Entry into the host cell breaching primary barriers- Pathogen invades host

using protein secretion systems. Gram negative pathogens have six different

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secretion systems named as Type I, Type II, Type III, Type IV, Type V and Type

VI. These secretions involve toxins, urease and multivalent adhesion molecule.

Toxins – These are small molecules made up of amino acids released by

pathogens and harmful for host after entry. These molecules influence the

normal process of body.

Effector proteins- These proteins help pathogen in breaching the primary

barrier of host, in degrading the host immunity and in some cases required for

the survival of pathogen.

Enzymes- One such enzyme is urease which is considered an important

virulence factor for some pathogens. For example- P. mirabilis can cause

urinary stone using urease.

b. Evasiveness of immune system of host by microbes

c. Replication of microbes in cells with or without spreading in the body

d. A host’s immunological abilities to evict or control the microbe

Figure 1: Steps in Host-Pathogen interaction

9. Basis for Host Pathogen Interactions:

There are several reasons that are responsible for the contact of host with pathogens. The

three factors involved are:

Type and mode of nutrition of pathogen-

All organisms require food for their survival and for energy. But there are different

modes for food intakes. Basically organisms are divided into two types; autotrophs

and heterotrophs. Autotrophs are those which can make their food by themselves; for

Invasion of the host

Evasion of host response

Multiplication of pathogen in the host

Host's immunological capability to control the pathogen

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example plants can make their food by trapping sunlight and taking water from soil.

These are further classified into photoautotroph and chemoautotroph. Photoautotroph

can make their food using energy from photon means from sun but chemoautotroph

uses energy from chemical reaction. These chemoautotrophs are present in oceanic

vents where sunlight cannot reach. Heterotrophs are the organisms which cannot

make their food by themselves. So these organisms eat other organisms for their food

and these are also classified into herbivores, carnivores and omnivores. Herbivores eat

plants and carnivores eat animals as their food. Omnivores can eat both plants as well

as animals as their food and these food link makes food chains. These different food-

chains make food-webs. Plants are the primary producer which makes food.

Herbivores are the primary consumers which eat plants.

Area needed by pathogen for shelter

Organisms require shelter for their existence. Parasites are the examples which cannot

exist without their host. So their host provides them shelter as well as food. Host

survival is most in this interaction because parasite can live only when his host is also

capable to survive.

Habits like breeding, aggregations.

Some organisms searches places for breeding that can protect their eggs from

predators. One such example of interaction is that fig plants need only wasp species

for pollination because wasp organisms lay their eggs in fig fruits. No other species

can replace wasp species. Other example is cukoo lays their eggs in crow’s nest and

all care is provided by crow. These habits of breeding are an example of interactions.

10. Machine learning based Host-Pathogen interaction

The exact mechanism of host-pathogen interaction can be elucidated by following methods:

a. Biological experiment based prediction

b. Homology based prediction

c. Structure based methods

d. Domain based prediction methods

Earlier host-pathogen interaction was studied only by biological experiment based prediction

methods. These methods are time consuming, cumbersome and labour intensive. Experiments

were performed in laboratory animals or model animals for human disease and the results

were extrapolated to human being. With the advancement in technology, bioinformatics tools

can be utilized for prediction of host-pathogen interaction based on homology and structure

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of protein involved. A number of databases for host-pathogen interaction have been

established. Important of them as follow:

i. PATRIC (PAThosystems Resource Integration Center)

ii. HPIDB (Host Pathogen Interaction Database)

iii. PIG (Pathway Interaction Gateway)

iv. VirHostNet

v. PHIDIAS – Pathogen Host Interaction Data Integration and Analysis System

vi. GPS-Prot

vii. VectorBase

viii. MvirDB

ix. PID

x. Viral Protein Structure Database

xi. EuPathDB

xii. BioHealthBase

xiii. PHI-base

11. Summary and future scopes

The host provides the pathogen with the necessary requirements and it depends on the

pathogen how to use the host mechanism for its benefits. All the diseases are the result of this

host pathogen interaction. Interactions can be positive or negative. Negative interactions are

responsible for evolution of protective methods in host. Hosts have evolved certain methods

to protect themselves from pathogens either it is secretion from specific cells or barrier

formed by cells. As the pathogens are capable of causing a disease in the host, it doesn’t

mean that they will cause disease in all the host cells. This resistance or susceptibility of host

towards a disease has been found to be genetic in nature. Scientists are trying to find out the

signature sequence indicating susceptibility to specific disease. Certain pathogens secrete

proteins which inhibit the macrophage action. It is an important aspect of host-pathogen

interaction. In-depth understanding of this aspect will help in designing drugs to augment the

working behaviour of macrophages. Human being also requires drugs to break the biofilm

formation as biofilm make the diffusion of antimicrobial compounds very slow and render

them ineffective. Host-pathogen interaction is rapidly emerging as an important area in

infectious disease research. These interactions become more complex when more than one

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pathogen infects the host. Elucidation of these complex interactions will pave the way for

discovery of new drugs and new treatment strategies for the diseases.

paper no. : 09 animal cell biotechnology module :21 host

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