Bacterial Diseases

Staphylococcus aureus can live harmlessly on the skin and mucous membranes of healthy people. When the skin barrier is broken, it can switch from a commensal organism to an opportunistic pathogen. It then uses surface adhesins, or attachment proteins, such as clumping factor B and Staphylococcus aureus surface protein G (SasG) to bind damaged epidermal proteins like loricrin and cytokeratin.

After attachment, S. aureus uses immune evasion tools to stay on the skin. Protein A binds the Fc...

Acne forms when pores in sebaceous skin become blocked and inflamed. It is a common skin condition in adolescents and young adults, and it often appears on the face, neck, chest, and back. The main lesions include comedones, papules, pustules, nodules, and, in severe cases, cysts.

The process starts with extra sebum production and follicular hyperkeratinization, which means the lining of the hair follicle sheds and thickens too much. These changes trap oil and keratin inside the follicle.

Bacillus anthracis is a Gram-positive, spore-forming bacterium that causes anthrax. It is a zoonotic disease, which means it can spread from animals to humans. Herbivorous animals are the main hosts, but people can become infected through skin contact, eating contaminated meat, or breathing in spores.

Cutaneous anthrax is the most common form. It usually starts when spores enter through small skin breaks, and it is generally less severe. Gastrointestinal anthrax comes from eating undercooked...

Rocky Mountain spotted fever is a serious tick-borne disease caused by Rickettsia rickettsii, a Gram-negative bacterium with a coccobacillary shape. It lives inside host cells, so it is an obligate intracellular parasite. Infection begins when an infected tick bites the skin and passes the bacterium to a person.

In the United States, the main tick vectors are Dermacentor variabilis, the American dog tick, and Dermacentor andersoni, the Rocky Mountain wood tick. Other tick species can also...

Bacterial meningitis is a severe infection that inflames the meninges, the protective membranes around the brain and spinal cord. It develops when pathogenic bacteria cross the blood-brain barrier and enter the cerebrospinal fluid (CSF), the fluid that surrounds the brain and spinal cord. Common causes include Neisseria meningitidis, Streptococcus pneumoniae, Haemophilus influenzae type b, Listeria monocytogenes, and Escherichia coli K1.

Pathogenic bacteria usually reach the meninges by...

Tetanus is a life-threatening neurological disorder that causes persistent muscle contractions and spastic paralysis. It is caused by Clostridium tetani, a motile, Gram-positive, rod-shaped, obligate anaerobe. The bacteria form terminal endospores, which give them a distinctive lollipop or tennis-racket appearance.

Botulism is a life-threatening disease caused by botulinum neurotoxin made by Clostridium botulinum, a Gram-positive, spore-forming, obligate anaerobe. Adults can develop botulism in different ways. In foodborne botulism, the body absorbs toxin that was already formed in food. In wound botulism, spores grow in injured tissue and release toxin into the blood.

Infant botulism works differently from adult botulism. It often begins after an infant swallows spores, such as those found in honey, and...

Plague is a serious zoonotic disease caused by Yersinia pestis, a Gram-negative, facultatively anaerobic coccobacillus. The bacterium mainly circulates in rodent populations. Humans usually become infected through the bite of infected fleas. It can also spread by direct contact with infected animal tissue or by inhaling respiratory droplets from a person with pneumonic plague.

Y. pestis uses several virulence factors to survive in the host. In the flea, it forms biofilms in the foregut that...

Strep throat is an acute infection of the oropharyngeal tissues caused by Group A Streptococcus, or Streptococcus pyogenes. It spreads mainly through respiratory droplets that are released when an infected person coughs, sneezes, or talks.

After the bacteria enter the body, they attach to the mucosal cells of the pharynx. Surface structures such as lipoteichoic acid and the antiphagocytic M protein help the bacteria stick to tissue. The M protein also blocks complement activation and...

Diphtheria is a toxin-mediated bacterial disease that usually affects the upper respiratory tract. It is caused by Corynebacterium diphtheriae, a Gram-positive, pleomorphic rod with a club-shaped form under the microscope. The bacteria can live on mucosal surfaces without causing symptoms. Clinical disease develops only when the strain is lysogenized by a specific β-corynephage that carries the tox gene for diphtheria toxin.

Diphtheria toxin is the main virulence factor, or disease-causing...

Atypical pneumonia is a lung infection that often causes walking pneumonia. It is commonly caused by Mycoplasma pneumoniae, a pleomorphic bacterium with no rigid cell wall. That missing cell wall makes it resistant to beta-lactam antibiotics and shapes how it behaves in the human body.

Other pathogens can also cause this disease. These include Legionella pneumophila, Coxiella burnetii, and Chlamydia psittaci. Each one is linked to different environmental or zoonotic exposures, which means the...

Tuberculosis is a lung infection caused by Mycobacterium tuberculosis, and it spreads through the air. When a person with active TB coughs or breathes out, tiny droplet nuclei can be inhaled by someone else. These particles reach the alveoli, the small air sacs in the lungs, where the bacteria are taken up by alveolar macrophages, a type of immune cell.

Mycobacterium tuberculosis can survive inside these phagocytes because of its lipid-rich cell wall. After phagocytosis, infected macrophages...

H. pylori and stomach ulcers are linked through damage to the stomach and duodenal lining. A peptic ulcer is an erosive lesion in the gastric or duodenal wall. The most common cause is Helicobacter pylori infection, a Gram-negative, helical bacterium that can live in the stomach’s acidic environment.

H. pylori survives acid by making urease, an enzyme that breaks urea into ammonia and carbon dioxide. The ammonia helps neutralize gastric acid around the bacterium, which supports colonization of...

Bacterial gastroenteritis is a gut infection that can cause diarrhea, abdominal cramps, and vomiting. It often begins after people ingest contaminated food or water. Pathogenic Escherichia coli strains are a frequent cause.

Some E. coli strains make disease by producing Shiga toxin. These are called Shiga toxin–producing E. coli, or STEC. A well-known example is O157:H7. The toxin binds to a host glycolipid receptor called globotriaosylceramide, or Gb3, and enters the cell through membrane...

Cholera is an acute gastrointestinal disease caused by the Gram-negative bacterium Vibrio cholerae. It spreads mainly through the fecal-oral route. People usually get infected by drinking contaminated water or eating contaminated food. The bacterium is motile and belongs to the Vibrionaceae family. It is linked to waterborne outbreaks in places with poor sanitation.

More than 200 serogroups of Vibrio cholerae exist, but only O1 and O139 cause epidemic cholera. The O1 serogroup, especially the...