Any disease that is caused by bacteria is known as bacterial disease. The earth is populated with several species of disease-causing bacteria that differ in their size, shape and disease-causing mechanism. Some bacteria may cause disease by excreting toxins, some may produce toxins internally or some may induce sensitivity to their antigenic properties. This article focuses on the epidemiology and characteristics of ten bacteria that cause diseases in humans.
1. Mycobacterium tuberculosis
Mycobacterium tuberculosis is a pathogenic bacteria belonging to the family Mycobacteriaceae that causes tuberculosis. It is a gram positive, aerobic bacteria that invades the mammalian respiratory system, specifically the lungs. The main reservoir of this bacterium is humans. It is usually understood as a communicable disease that spreads through human contact but it is spread through air droplets that are produced by an infected person by sneezing or coughing.
M. tuberculosis was first discovered by Robert Koch in 1882. They are known to have a waxy surface coating of mycolic acid which makes them impervious to Gram staining and hence the bacteria stains lightly and appears weakly gram-positive.
2. Corynebacterium diphtheriae
Corynebacterium diphtheriae is a pathogenic bacteria that causes diphtheria in humans. This bacteria was first identified by Edwin Klebs and Friedrich Löffler and are hence also called as Klebs-Löffler bacillus. It is a non-motile, rod-shaped, gram-positive bacteria. It affects the upper respiratory system, specifically the nasopharyngeal region which makes breathing and swallowing difficult.
The bacteria enter into the human body through the nose, throat and tonsils. It forms a thick, grey coating in the nasopharynx region and remains contagious for upto two weeks. It spreads by respiratory droplets such as coughing and sneezing but can also spread through spores or open, contaminated surfaces. If left untreated, it can badly affect the heart, kidney and nerves.
3. Vibrio cholerae
Vibrio cholerae is a gram-negative, comma-shaped and facultative anaerobic bacteria. While they are commonly found attached to shells of crabs and shrimps in saltwater, some strains are disease causing in humans and cause cholera. It was first identified by Félix-Archimède Pouchet in 1849 as a form of protozoa, but later Filippo Pacini correctly described it as a bacteria. The bacterium possesses flagella at one end and several pili on the whole cell surface.
V. cholerae affects the intestines by releasing cholera toxin that causes profuse diarrhoea. The infection is usually picked by consuming contaminated food and water or coming in contact with human faeces. Major symptoms of the disease include vomiting, low blood pressure, diarrhoea and muscle cramps.
4. Mycobacterium leprae
Mycobacterium leprae is an aerobic, disease-causing bacteria. It is a rod-shaped bacteria that has a unique waxy coating, characteristic of the Mycobacteriaceae family. It does not stain with Gram stain but with carbol fuchsin. It causes leprosy, also known as Hansen’s disease. It is a chronic, infectious but curable disease that affects the peripheral nervous system of an organism, mainly the eyes, nose, skin and muscles.
The bacterium was first discovered by Gerhard Armauer Hansen in 1873. It invades the nerve cells of an organism and replicates rapidly, colonising the Schwann cells. They then cause giant infiltrations in the skin causing dry, pale and flattened patches on the skin.
5. Bordetella pertussis
Bordetella pertussis is a gram-negative, aerobic coccobacilli that causes pertussis or whooping cough. It possesses a flagella-like structure and is motile in nature. Humans are the only reservoir of this bacterium and are spread by air droplets. The bacteria was first identified by Guillaume de Baillou in 1578.
In pertussis, the respiratory system is affected, producing a whooping sound during coughing. B. pertussis colonises the epithelial cells of the lungs. The bacteria releases a protein that binds to the cilia of the epithelial cells and prevents the cilia from beating. This leads to collection of debris in the lungs which is signalled by the whooping cough in the host.
6. Clostridium tetani
Clostridium tetani is a rod-shaped, gram-positive bacterium that causes tetanus. It is a soil-dwelling bacteria that possesses various flagella and is motile in nature. It is 2.5 μm long but becomes enlarged into tennis or drumstick shapes after forming spores. The spores are hard and can be found in soil and the gastrointestinal tract of animals.
The C.tetani spores can enter into our body through open wounds. Upon entering they release toxins in the body which travel via the bloodstream and lymphatic system into the whole body and block the neurotransmitters. This blockage leads to multiple muscle spasms in the body followed by lockjaw and abdominal spasms.
7. Yersinia pestis
Yersinia pestis, formerly known as Pasteurella pestis, is a non-motile, gram-negative and coccobacilli bacteria that causes plague in humans. It is a facultative anaerobic organism that infects humans by the vector Oriental rat flea. The plague disease is associated with the deadliest pandemic recorded in history as it caused the first plague pandemic and Black Death.
Y. pestis was first identified by Alexandre Yersin. It is transmitted into humans by fleas. Fleas acquire the bacterium by feeding on an infected animal. It is then transferred into humans when fleas feed on human blood. The bacteria then reaches into the oesophagus via blood and dislodges the bacteria residing there.
8. Neisseria gonorrhoeae
Neisseria gonorrhoeae is a Gram-negative diplococcus bacteria that is also known as gonococcus or gonococci. It was first described by Albert Neisser in 1879. It causes gonorrhoea, a genitourinary infection that is sexually transmitted.
Symptoms of gonorrhoea include increased urge to urinate, burning during urination and a pus-like discharge from the penis in men. The infection can spread to the testicles and prostate, if left untreated. In women, the primary symptoms include vaginal discharge, menstrual abnormalities and burning during urination.
9. Treponema pallidum
Treponema pallidum is a motile spirochaete bacterium that causes syphilis in humans. It is also known to cause a condition in cattles, bovine digital thermalis. It is a helically coiled bacterium that is 6–15 μm long and 0.1–0.2 μm wide. It is considered a Gram-negative bacteria, however it lacks lipopolysaccharide in its walls which is a characteristic of Gram-negative bacteria.
Syphilis is caused by sexual contact and the bacterium breaches the coulmnar or squamous epithelium cells. It causes rashes or lesions on the genital area of men and women which gets worse with scratching. Other symptoms include fever, swollen lymph nodes and muscle pain.
10. Salmonella enteritidis
Salmonella enteritidis is a gram-negative, facultative anaerobe that is a member of the family Enterobacteriaceae. They are motile, spore forming bacteria that cause salmonellosis in humans. It is mainly found in the intestinal tract of animals and spreads by eating contaminated poultry meat, eggs and milk. The symptoms of salmonellosis include diarrhoea, fever and abdominal pain.
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