Pathogenic bacteria
Neisseria gonorrhoeae (small red dots) inpus from a man with aurethral discharge (Gram stain)

Pathogenic bacteria arebacteria that can causedisease.^[1] This article focuses on the bacteria that arepathogenic to humans. Mostspecies of bacteria are harmless and many arebeneficial but others can causeinfectious diseases. The number of these pathogenic species in humans is estimated to be fewer than a hundred.^[2] By contrast, several thousand species are considered part of thegut flora, with a few hundred species present in each individual human'sdigestive tract.^[3]

The body is continually exposed to many species of bacteria, including beneficialcommensals, which grow on the skin andmucous membranes, andsaprophytes, which grow mainly in the soil and indecaying matter. The blood and tissue fluids contain nutrients sufficient to sustain the growth of many bacteria. The body has defence mechanisms that enable it to resist microbial invasion of its tissues and give it a naturalimmunity orinnate resistance against manymicroorganisms.

Pathogenic bacteria are specially adapted and endowed with mechanisms for overcoming the normal body defences, and can invade parts of the body, such as the blood, where bacteria are not normally found. Some pathogens invade only the surfaceepithelium, skin or mucous membrane, but many travel more deeply, spreading through the tissues and disseminating by thelymphatic andblood streams. In some rare cases a pathogenic microbe can infect an entirely healthy person, but infection usually occurs only if the body's defence mechanisms are damaged by some local trauma or an underlying debilitating disease, such as wounding,intoxication,chilling, fatigue, andmalnutrition. In many cases, it is important to differentiateinfection andcolonization, which is when the bacteria are causing little or no harm.

Caused byMycobacterium tuberculosis bacteria, one of thediseases with the highestdisease burden istuberculosis, which killed 1.4 million people in 2019, mostly insub-Saharan Africa.^[5] Pathogenic bacteria contribute to other globally important diseases, such aspneumonia, which can be caused by bacteria such asStaphylococcus,Streptococcus andPseudomonas, andfoodborne illnesses, which can be caused by bacteria such asShigella,Campylobacter, andSalmonella. Pathogenic bacteria also cause infections such astetanus,typhoid fever,diphtheria,syphilis, andleprosy.

Pathogenic bacteria are also the cause of highinfant mortality rates indeveloping countries.^[6] AGBD study estimated theglobal death rates from (33) bacterial pathogens, finding such infections contributed to one in 8 deaths (or ~7.7 million deaths), whichcould make it thesecond largest cause of death globally in 2019.^[7][4]

Most pathogenic bacteria can be grown incultures and identified byGram stain and other methods. Bacteria grown in this way are oftentested to find whichantibiotics will be an effective treatment for the infection. For hitherto unknown pathogens,Koch's postulates are the standard to establish acausative relationship between a microbe and a disease.

Each species has specific effect and causes symptoms in people who are infected. Some people who are infected with a pathogenic bacteria do not have symptoms. Immunocompromised individuals are more susceptible to pathogenic bacteria.^[8]

Some pathogenic bacteria cause disease under certain conditions, such as entry through the skin via a cut, through sexual activity or through compromised immune function.^{[citation needed]}

Some species ofStreptococcus andStaphylococcus are part of the normalskin microbiota and typically reside on healthy skin or in the nasopharyngeal region. Yet these species can potentially initiate skin infections.Streptococcal infections includesepsis,pneumonia, andmeningitis.^[9] These infections can become serious creating a systemic inflammatory response resulting in massive vasodilation, shock, and death.^[10]

Other bacteria areopportunistic pathogens and cause disease mainly in people withimmunosuppression orcystic fibrosis. Examples of these opportunistic pathogens includePseudomonas aeruginosa,Burkholderia cenocepacia, andMycobacterium avium.^[11][12]

Obligate intracellular parasites (e.g.Chlamydophila,Ehrlichia,Rickettsia) are only able to grow and replicate inside other cells. Infections due to obligate intracellular bacteria may beasymptomatic, requiring anincubation period. Examples of obligate intracellular bacteria includeRickettsia prowazekii (typhus) andRickettsia rickettsii, (Rocky Mountain spotted fever).^{[citation needed]}

Chlamydia are intracellular parasites. These pathogens can causepneumonia orurinary tract infection and may be involved incoronary heart disease.^[13]

Other groups of intracellular bacterial pathogens includeSalmonella,Neisseria,Brucella,Mycobacterium,Nocardia,Listeria,Francisella,Legionella, andYersinia pestis. These can exist intracellularly, but can exist outside host cells.^{[citation needed]}

Bacterial pathogens often cause infection in specific areas of the body. Others are generalists.

• Bacterial vaginosis is a condition of thevaginal microbiota in which an excessive growth ofGardnerella vaginalis and other mostlyanaerobic bacteria displace the beneficialLactobacilli species that maintain healthy vaginal microbial populations.^[14]
• Bacterial meningitis is a bacterialinflammation of themeninges, which are the protective membranes covering thebrain andspinal cord.
• Bacterial pneumonia is a bacterial infection of thelungs.
• Urinary tract infection is predominantly caused by bacteria. Symptoms include the strong and frequent sensation or urge to urinate, pain duringurination, and urine that is cloudy.^[15] The most frequent cause isEscherichia coli.Urine is typically sterile but contains a variety of salts and waste products. Bacteria can ascend into thebladder orkidney and causingcystitis andnephritis.^[16][17]
• Bacterial gastroenteritis is caused by enteric, pathogenic bacteria. These pathogenic species are usually distinct from the usually harmless bacteria of the normalgut flora. But a different strain of the same species may be pathogenic. The distinction is sometimes difficult as in the case ofEscherichia.
• Bacterialskin infections include:
  • Impetigo is a highly contagiousbacterialskininfection commonly seen in children.^[18] It is caused byStaphylococcus aureus, andStreptococcus pyogenes.^[19]
  • Erysipelas is an acutestreptococcusbacterial infection^[20] of the deeper skin layers that spreads via with lymphatic system.
  • Cellulitis is a diffuseinflammation^[21] ofconnective tissue with severe inflammation of dermal and subcutaneous layers of theskin. Cellulitis can be caused by normalskin flora or bycontagious contact, and usually occurs through open skin, cuts,blisters, cracks in the skin,insect bites,animal bites,burns,surgical wounds, intravenousdrug injection, or sites ofintravenouscatheter insertion. In most cases it is the skin on the face or lower legs that is affected, though cellulitis can occur in other tissues.

The symptoms of disease appear as pathogenic bacteria damage host tissues or interfere with their function. The bacteria can damage host cells directly or indirectly by provoking an immune response that inadvertently damages host cells,^[22] or by releasingtoxins.^[23]

Once pathogens attach to host cells, they can cause direct damage as the pathogens use the host cell for nutrients and produce waste products.^[24] For example,Streptococcus mutans, a component ofdental plaque, metabolizes dietary sugar and produces acid as a waste product. The acid decalcifies the tooth surface to causedental caries.^[25]

Endotoxins are the lipid portions of lipopolysaccharides that are part of the outer membrane of the cell wall ofgram-negative bacteria. Endotoxins are released when the bacterialyses, which is why after antibiotic treatment, symptoms can worsen at first as the bacteria are killed and they release their endotoxins.Exotoxins are secreted into the surrounding medium or released when the bacteria die and the cell wall breaks apart.^[26]

An excessive or inappropriate immune response triggered by an infection may damage host cells.^[1]

Iron is required for humans, as well as the growth of most bacteria. To obtain free iron, some pathogens secrete proteins calledsiderophores, which take the iron away from iron-transport proteins by binding to the iron even more tightly. Once the iron-siderophore complex is formed, it is taken up by siderophore receptors on the bacterial surface and then that iron is brought into the bacterium.^[26]

Bacterial pathogens also require access to carbon and energy sources for growth. To avoid competition with host cells for glucose which is the main energy source used by human cells, many pathogens including therespiratory pathogenHaemophilus influenzae specialise in using other carbon sources such aslactate that are abundant in the human body^[27]

Typically identification is done by growing the organism in a wide range of cultures which can take up to 48 hours. The growth is then visually or genomically identified. The cultured organism is then subjected to various assays to observe reactions to help further identify species and strain.^[28]

Bacterial infections may be treated withantibiotics, which are classified asbacteriocidal if they kill bacteria orbacteriostatic if they just prevent bacterial growth. There are many types of antibiotics and each classinhibits a process that is different in the pathogen from that found in the host. For example, the antibioticschloramphenicol andtetracyclin inhibit the bacterialribosome but not the structurally different eukaryotic ribosome, so they exhibit selective toxicity.^[29] Antibiotics are used both in treating human disease and inintensive farming to promote animal growth. Both uses may be contributing to the rapid development ofantibiotic resistance in bacterial populations.^[30]Phage therapy, usingbacteriophages can also be used to treat certain bacterial infections.^[31]

Infections can be prevented byantiseptic measures such as sterilizing the skin prior to piercing it with the needle of a syringe and by proper care of indwelling catheters. Surgical and dental instruments are alsosterilized to prevent infection by bacteria.Disinfectants such asbleach are used to kill bacteria or other pathogens on surfaces to prevent contamination and further reduce the risk of infection. Bacteria in food are killed by cooking to temperatures above 73 °C (163 °F).^{[citation needed]}

Manygenera contain pathogenic bacterialspecies. They often possess characteristics that help to classify and organize them into groups. The following is a partial listing.

• 
  Overall age-standardised mortality rate per 100 000 population for 33 pathogens investigated, 2019^[4]
• 
  Global number of deaths (A) and YLLs (B), by pathogen and infectious syndrome, 2019^[4]
• 
  Global number of deaths, by pathogen, age, and sex groups, 2019^[4]

This is description of the more common genera and species presented with their clinical characteristics and treatments.

Of the 59 species listed in the table with their clinical characteristics, 11 species (or 19%) are known to be capable of naturalgenetic transformation.^[82] Natural transformation is a bacterial adaptation for transferringDNA from one cell to another. This process includes the uptake of exogenous DNA from a donor cell by a recipient cell and its incorporation into the recipient cell'sgenome byrecombination. Transformation appears to be an adaptation for repairingdamage in the recipient cell's DNA. Among pathogenic bacteria, transformation capability likely serves as an adaptation that facilitates survival and infectivity.^[82] The pathogenic bacteria able to carry out natural genetic transformation (of those listed in the table) areCampylobacter jejuni,Enterococcus faecalis,Haemophilus influenzae,Helicobacter pylori,Klebsiella pneumoniae,Legionella pneumophila,Neisseria gonorrhoeae,Neisseria meningitidis,Staphylococcus aureus,Streptococcus pneumoniae andVibrio cholerae.^{[citation needed]}

• Human microbiome project
• List of antibiotics
• Pathogenic viruses