Pathogenic bacteria

Pathogenic bacteria are specific types of bacteria that cause disease in humans. While most bacteria are harmless or even beneficial, these pathogens are specially adapted to overcome the body's natural defenses. For example, the body has an innate resistance to many microorganisms. However, pathogens can invade areas where bacteria are not normally found, such as the blood or tissue fluids.

It is important to distinguish between infection and colonization. Colonization occurs when bacteria are present but cause little to no harm to the host.

These bacteria damage the host through several different mechanisms. Some pathogens cause direct damage by attaching to host cells and using them for nutrients. An example is Streptococcus mutans, which lives in dental plaque. It metabolizes dietary sugar and produces acid as a waste product, which decalcifies teeth to cause dental caries. Other bacteria produce toxins, which are poisonous substances. Endotoxins are lipid portions of lipopolysaccharides found in the outer membrane of Gram-negative bacteria. These are released when the bacteria lyse, or break apart. Exotoxins are secreted into the surrounding medium or released when the cell wall breaks.

Finally, an infection can cause indirect damage. This happens when an excessive or inappropriate immune response inadvertently damages the host's own cells.

Pathogens vary in how they interact with human cells and tissues. Some are obligate intracellular parasites, such as Rickettsia prowazekii, which causes typhus. These organisms can only grow and replicate inside other cells. Other bacteria, such as Salmonella or Listeria, can exist both inside and outside host cells. Some bacteria are opportunistic pathogens. These mainly cause disease in people with immunosuppression or conditions like cystic fibrosis. Examples include Pseudomonas aeruginosa and Burkholderia cenocepacia. Other species might be part of the normal skin microbiota, like certain Staphylococcus species, but can initiate infections if conditions change.

Bacterial infections can target specific parts of the body or act as generalists. For instance, bacterial meningitis is an inflammation of the meninges, which are the protective membranes covering the brain and spinal cord. Bacterial pneumonia specifically affects the lungs. Urinary tract infections are often caused by Escherichia coli, which can ascend into the bladder or kidney to cause cystitis or nephritis. Some bacteria cause skin-specific issues. Impetigo is a highly contagious infection caused by Staphylococcus aureus or Streptococcus pyogenes. Erysipelas is an acute infection of deeper skin layers that spreads via the lymphatic system. Cellulitis is a more diffuse inflammation of connective tissue and skin layers.

To survive inside a human, bacteria must compete for essential resources. Iron is required for the growth of most bacteria. To obtain free iron, some pathogens secrete proteins called siderophores. These proteins bind to iron more tightly than the host's own transport proteins. Once the iron-siderophore complex forms, the bacterium takes it up through surface receptors. Bacteria also need carbon and energy sources. To avoid competing with human cells for glucose, some pathogens, like Haemophilus influenzae, specialize in using other carbon sources like lactate.

Pathogenic bacteria have a massive impact on global health. A GBD study estimated that bacterial pathogens contributed to one in eight deaths globally in 2019. This accounts for approximately 7.7 million deaths, making them the second largest cause of death globally that year.

One specific example is Mycobacterium tuberculosis, which caused 1.4 million deaths in 2019, mostly in sub-Saharan Africa. Other bacteria cause significant disease burdens through pneumonia, foodborne illnesses, or infections like tetanus, typhoid fever, and diphtheria.

Medical professionals use several methods to identify and treat these organisms. Bacteria can be grown in cultures, which may take up to 48 hours, and then identified visually or genomically. Doctors also use a Gram stain to help classify them. Once identified, antibiotics are used for treatment. Antibiotics are classified as bacteriocidal if they kill bacteria, or bacteriostatic if they only prevent growth. Some antibiotics, like chloramphenicol, exhibit selective toxicity by targeting bacterial ribosomes while leaving human eukaryotic ribosomes alone.

Prevention is also possible through sterilization of medical tools, using disinfectants like bleach, and cooking food to temperatures above 73 °C (163 °F).