Biology Salmonella typhi

Salmonella typhi is a pathogenic bacterium which causes typhoid

fever in human beings. These pathogens generally enter the small intestine

through food and water contaminated with them and migrate to other

organs through blood. Sustained high fever (39° to 40°C), weakness,

stomach pain, constipation, headache and loss of appetite are some of

the common symptoms of this disease. Intestinal perforation and death

may occur in severe cases.

Typhoid fever could be confirmed byWidal test. A classic case in medicine, that of Mary Mallon nicknamed

Typhoid Mary, is worth mentioning here. She was a cook by profession

and was a typhoid carrier who continued to spread typhoid for several

years through the food she prepared.

Bacteria like Streptococcus pneumoniae and Haemophilus influenzae

are responsible for the disease pneumonia in humans which infects the

alveoli (air filled sacs) of the lungs. As a result of the infection, the alveoli

get filled with fluid leading to severe problems in respiration. The symptoms

of pneumonia include fever, chills, cough and headache. In severe cases,

the lips and finger nails may turn gray to bluish in colour. A healthy

person acquires the infection by inhaling the droplets/aerosols released

by an infected person or even by sharing glasses and utensils with an

infected person. Dysentery, plague, diphtheria, etc., are some of the other

bacterial diseases in man.

Many viruses also cause diseases in human beings. Rhino viruses

represent one such group of viruses which cause one of the most infectious

human ailments – the common cold. They infect the nose and respiratory

passage but not the lungs. The common cold is characterised by nasal

congestion and discharge, sore throat, hoarseness, cough, headache,

tiredness, etc., which usually last for 3-7 days. Droplets resulting from

cough or sneezes of an infected person are either inhaled directly or

transmitted through contaminated objects such as pens, books, cups,

doorknobs, computer keyboard or mouse, etc., and cause infection in a

healthy person.

Some of the human diseases are caused by protozoans too. You might

have heard about malaria, a disease man has been fighting since many

years. Plasmodium, a tiny protozoan is responsible for this disease. Different

species of Plasmodium (P. vivax, P. malaria and P. falciparum) are

responsible for different types of malaria. Of these, malignant malaria caused by Plasmodium falciparum is the most serious one and can even be fatal.

Let us take a glance at the life cycle of Plasmodium (Figure 8.1).

Plasmodium enters the human body as sporozoites (infectious form)

through the bite of infected female Anopheles mosquito. The parasites

initially multiply within the liver cells and then attack the red blood cells

(RBCs) resulting in their rupture. The rupture of RBCs is associated with

release of a toxic substance, haemozoin, which is responsible for the chill

and high fever recurring every three to four days. When a female Anopheles

mosquito bites an infected person, these parasites enter the mosquito’s

body and undergo further development. The parasites multiply within

them to form sporozoites that are stored in their salivary glands. When

these mosquitoes bite a human, the sporozoites are introduced into his/

her body, thereby initiating the events mentioned above. It is interesting

to note that the malarial parasite requires two hosts – human and

mosquitoes – to complete its life cycle (Figure 8.1); the female Anopheles

mosquito is the vector (transmitting agent) too

Entamoeba histolytica is a protozoan parasite in the large intestine of

human which causes amoebiasis (amoebic dysentery). Symptoms of

this disease include constipation, abdominal pain and cramps, stools

with excess mucous and blood clots. Houseflies act as mechanical carriers

and serve to transmit the parasite from faeces of infected person to food

and food products, thereby contaminating them.

Drinking water and food contaminated by the faecal

matter are the main source of infection.

Ascaris, the common round worm and Wuchereria,

the filarial worm, are some of the helminths which are

known to be pathogenic to man. Ascaris, an intestinal

parasite causes ascariasis. Symptoms of these disease

include internal bleeding, muscular pain, fever, anemia

and blockage of the intestinal passage. The eggs of the

parasite are excreted along with the faeces of infected

persons which contaminate soil, water, plants, etc. A

healthy person acquires this infection through

contaminated water, vegetables, fruits, etc.

Wuchereria (W. bancrofti and W. malayi), the filarial

worms cause a slowly developing chronic inflammation

of the organs in which they live for many years, usually

the lymphatic vessels of the lower limbs and the disease

is called elephantiasis or filariasis (Figure 8.2). The

genital organs are also often affected, resulting in gross

deformities. The pathogens are transmitted to a healthy

person through the bite by the female mosquito vectors.

Many fungi belonging to the genera Microsporum,

T richophyton and Epidermophyton are

responsible for ringworms which is one of

the most common infectious diseases in man.

Appearance of dry, scaly lesions on various

parts of the body such as skin, nails and

scalp (Figure 8.3) are the main symptoms of

the disease. These lesions are accompanied

by intense itching. Heat and moisture help

these fungi to grow, which makes them thrive

in skin folds such as those in the groin or

between the toes. Ringworms are generally

acquired from soil or by using towels, clothes

or even the comb of infected individuals.

The principle of immunisation or vaccination is based on the property of

‘memory’ of the immune system.

In vaccination, a preparation of antigenic

proteins of pathogen or inactivated/weakened pathogen (vaccine) are

introduced into the body. The antibodies produced in the body against

these antigens would neutralise the pathogenic agents during actual

infection. The vaccines also generate memory – B and T-cells that recognise

the pathogen quickly on subsequent exposure and overwhelm the

invaders with a massive production of antibodies. If a person is infected

with some deadly microbes to which quick immune response is required

as in tetanus, we need to directly inject the preformed antibodies, or

antitoxin (a preparation containing antibodies to the toxin). Even in cases

of snakebites, the injection which is given to the patients, contain preformed

antibodies against the snake venom. This type of immunisation is called

passive immunisation.

Recombinant DNA technology has allowed the production of antigenic

polypeptides of pathogen in bacteria or yeast. Vaccines produced using

this approach allow large scale production and hence greater availability

for immunisation, e.g., hepatitis B vaccine produced from yeast.

AIDS is caused by the Human Immuno deficiency Virus (HIV), a

member of a group of viruses called retrovirus, which have an envelope

enclosing the RNA genome (Figure 8.6). Transmission of HIV-infection

generally occurs by (a) sexual contact with infected person, (b) by

transfusion of contaminated blood and blood products, (c) by sharing

infected needles as in the case of intravenous drug abusers and (d) from

infected mother to her child through placenta.

Simultaneously, HIV enters into helper

T-lymphocytes (TH), replicates and produce progeny viruses. The progeny

viruses released in the blood attack other helper T-lymphocytes. This is repeated leading to a progressive decrease in the number of  helper T-lymphocytes in the body of the infected person. During this period , the person suffers from bouts of fever, diarrhoea and weight loss.

Due to decrease in

the number of helper T lymphocytes, the person starts suffering from infections

that could have been otherwise overcome such as those due to bacteria

especially Mycobacterium, viruses, fungi and even parasites like Toxoplasma.

The patient becomes so immuno-deficient that he/she is unable to protect

himself/herself against these infections

A widely used diagnostic test for

AIDS is enzyme linked immuno-sorbent assay (ELISA). Treatment of AIDS

with anti-retroviral drugs is only partially effective. They can only prolong

the life of the patient but cannot prevent death, which is inevitable

In our body, cell growth and differentiation is highly controlled and

regulated. In cancer cells, there is breakdown of these regulatory

mechanisms. Normal cells show a property called contact inhibition by

virtue of which contact with other cells inhibits their uncontrolled growth.

Cancer cells appears to have lost this property. As a result of this, cancerous

cells just continue to divide giving rise to masses of cells called tumors.

Tumors are of two types: benign and malignant. Benign tumors normally

remain confined to their original location and do not spread to other parts

of the body and cause little damage. The malignant tumors, on the

other hand are a mass of proliferating cells called neoplastic or tumor

cells. These cells grow very rapidly, invading and damaging the

surrounding normal tissues. As these cells actively divide and grow they

also starve the normal cells by competing for vital nutrients. Cells sloughed

from such tumors reach distant sites through blood, and wherever they

get lodged in the body, they start a new tumor there. This property called

metastasis is the most feared property of malignant tumors.

Causes of cancer :

Transformation of normal cells into cancerous

neoplastic cells may be induced by physical, chemical or biological agents.

These agents are called carcinogens. Ionising radiations like X-rays and

gamma rays and non-ionizing radiations like UV cause DNA damage

leading to neoplastic transformation. The chemical carcinogens present

in tobacco smoke have been identified as a major cause of lung cancer.

Cancer causing viruses called oncogenic viruses have genes called viral

oncogenes. Furthermore, several genes called cellular oncogenes

(c-onc) or proto oncogenes have been identified in normal cells which,

when activated under certain conditions, could lead to oncogenic

transformation of the cells

Health is not just the absence of disease. It is a state of complete physical,

mental, social and psychological well-being. Diseases like typhoid,

cholera, pneumonia, fungal infections of skin, malaria and many others

are a major cause of distress to human beings. Vector-borne diseases

like malaria especially one caused by Plasmodium falciparum, if not

treated, may prove fatal. Besides personal cleanliness and hygiene,

public health measures like proper disposal of waste, decontamination

of drinking water, control of vectors like mosquitoes and immunisation

are very helpful in preventing these diseases. Our immune system plays

the major role in preventing these diseases when we are exposed to

disease-causing agents. The innate defences of our body like skin,

mucous membranes, antimicrobial substances present in our tears,

saliva and the phagocytic cells help to block the entry of pathogens

into our body. If the pathogens succeed in gaining entry to our body,

specific antibodies (humoral immune response) and cells (cell mediated

immune response) serve to kill these pathogens. Immune system has

memory. On subsequent exposure to same pathogen, the immune

response is rapid and more intense. This forms the basis of protection

afforded by vaccination and immunisation. Among other diseases, AIDS

and cancer kill a large number of individuals worldwide. AIDS caused

by the human immuno-deficiency virus (HIV) is fatal but can be

prevented if certain precautions are taken. Many cancers are curable if

detected early and appropriate therapeutic measures are taken. Of late,

drug and alcohol abuse among youth and adolescents is becoming

another cause of concern. Because of the addictive nature of alcohol

and drugs, and their perceived benefits like relief from stress, a person

may try taking these in the face of peer pressure, examinations-related

and competition-related stresses. In doing so, he/she may get addicted

to them. Education about their harmful effects, counselling and seeking

immediate professional and medical help would totally relieve the

individual from these evils.

What are the various public health measures, which you would suggest

as safeguard against infectious diseases?

1. In which way has the study of biology helped us to control infectious

diseases?

2. How does the transmission of each of the following diseases take place?

(a) Amoebiasis (b) Malaria (c) Ascariasis (d) Pneumonia

3. What measure would you take to prevent water-borne diseases?

4. Discuss with your teacher what does ‘a suitable gene’ means, in the

context of DNA vaccines