Each time you take antibiotics, you increase the chances that bacteria in your body will learn to resist them causing antibiotic resistance. Later, you could get or spread an infection that those antibiotics cannot cure. The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health. Bacterial Infections. Learn More Related Issues Specifics. See, Play and Learn Images.
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Both Lyme disease and Legionnaire's disease, now well-known to health-care professionals, were discovered as recently as the s. The recent increased prevalence of highly immunosuppressed individuals, both due to AIDS and the increasing use of immunosuppressive drugs as chemotherapy and for transplantation of organs, tissues, and cells, has led to a population of patients highly susceptible to types of bacterial infections that were comparatively rare before. Several factors lead to the development of bacterial infection and disease.
First, the infectivity of an organism determines the number of individuals that will be infected compared to the number who are susceptible and exposed. Second, the pathogenicity is a measure of the potential for an infectious organism to cause disease. Pathogenic bacteria possess characteristics that allow them to evade the body's protective mechanisms and use its resources, causing disease.
Finally, virulence describes the organism's propensity to cause disease, through properties such as invasiveness and the production of toxins. Host factors are critical in determining whether disease will develop following transmission of a bacterial agent.
These factors include genetic makeup, nutritional status, age, duration of exposure to the organism, and coexisting illnesses. The environment also plays a role in host susceptibility. Air pollution as well as chemicals and contaminants in the environment weaken the body's defenses against bacterial infection. Bacteria are prokaryotic organisms that carry their genetic information in a double-stranded circular molecule of DNA.
Some species also contain small circular plasmids of additional DNA. The cell cytoplasm contains ribosomes and there is both a cell membrane and, in all species except Mycoplasma , a complex cell wall. External to the cell wall, some bacteria have capsules, flagella, or pili see Figure 1.
Bacteria normally reproduce by binary fission. Under the proper conditions, some bacteria can divide and multiply rapidly. Consequently, some infections require only a small number of organisms to cause potentially overwhelming infection. Structure of a bacterium. In: Infectious Disease , 2nd edn. Bacteria are classified as Gram-positive or Gram-negative based on the characteristics of their cell wall, as seen under a microscope after stains have been administered, a procedure called Gram staining, that was developed in by Hans Christian Gram see Figure 2.
Most, but not all, bacteria fall into one of these two categories. Clinically, one of the main differences between gram-positive and gram-negative organisms is that gram-negative bacteria tend to produce an endotoxin that can cause tissue destruction, shock, and death. The two classes of bacteria differ in their antibiotic susceptibilities as well.
Bacteria can also be classified based on their growth responses in the presence and absence of oxygen. Aerobic bacteria, or aerobes, grow in the presence of oxygen. Obligate aerobes such as Bordetella pertussis require oxygen. Facultative organisms can grow in the presence or absence of oxygen. Anaerobic bacteria such as the Clostridia are able to grow in the absence of oxygen and obligate anaerobes require its absence.
Some bacteria are not classified as Gram-positive or Gram-negative. These include the mycobacteria, of which Mycobacterium tuberculosis is the most well-known, which can be seen under the microscope using a special stain called the acid-fast stain; organisms that do not take up Gram stain such as the spirochetes which cause diseases such as syphilis and Lyme disease ; and the Rickettsia which cause Rocky Mountain spotted fever and epidemic typhus.
All of the human organs are susceptible to bacterial infection. Each species of bacteria has a predilection to infect certain organs and not others. For example, Neisseria meningitidis normally infects the meninges covering of the central nervous system, causing meningitis, and can also infect the lungs, causing pneumonia.
It is not, however, a cause of skin infection. Staphylococcus aureus , which people typically carry on their skin or mucus membranes, often causes skin and soft tissue infections, but also spreads readily throughout the body via the bloodstream and can cause infection of the lungs, abdomen, heart valves, and almost any other site.
Disease can be caused by destruction of the body's cells by the organism or the body's immune response to the infection. Antibiotics may be of little or no use when the disease manifestations are a result of the body's attempts to rid itself of the bacteria.
The systemic inflammatory response syndrome SIRS , usually caused by a bacterial infection, is an overwhelming inflammatory response to infection, manifested by the release of large numbers of cytokines and presenting with signs of infection and early signs of hemodynamic instability.
If allowed to progress, SIRS patients can go on to develop sepsis, with multiorgan failure and death. Once the cascade of events has begun, even the strongest antibiotics are often powerless to stop this progression. The external environment is usually the setting in which the bacterial agent and the host interact and the infection is acquired. Bacteria can be transmitted to humans through air, water, food, or living vectors. The macro- or microenvironments can also be thought of as playing a role in the spread of bacteria.
Certain settings such as hospitals and prisons harbor specific types of organisms. Some bacteria are endemic in certain geographic regions and rare or nonexistent in others. A reservoir is any site where a pathogen can survive until its transfer to a host. Often pathogens multiply within their reservoirs. Some reservoirs are living. Humans, animals, birds, and arthropods are all common reservoirs and do not always manifest illness due to the pathogen they are harboring.
Nonliving reservoirs include food, air, soil, and water. Fomites are inanimate objects capable of transmitting infection. Humans are the reservoirs for many bacterial infections and in some instances they are the exclusive host in nature to harbor the bacteria. When a human is colonized with a pathogen without manifesting disease, he or she is referred to as a carrier.
Passive carriers carry pathogens without ever having the disease. The deadly meningitis caused by Neisseria meningitidis is often transmitted by passive carriers who harbor the bacteria in their respiratory tracts.
An incubatory carrier is a person who is harboring, and can transmit, an infection during the incubation period the time between acquisition and manifestation of illness for that infection. Sexually transmitted infections are frequently transferred by individuals who have not yet shown symptoms.
Convalescent carriers manifested symptoms of an infectious disease in the recent past and continue to carry the organism during their recovery period. Active carriers have completely recovered from a disease and harbor the organism indefinitely. Salmonella , especially Salmonella Typhi, the cause of typhoid fever, is an example of a bacterial infection that can produce a prolonged carrier state without the individual being aware of the condition.
Salmonella can lurk in a quiescent state in organs such as the gallbladder, sometimes even permanently. These individuals may continuously transfer the pathogen to their contacts.
Mary Mallon, a New York City cook in the early s, known as Typhoid Mary, was a carrier responsible for many cases of typhoid fever. Infections acquired from animal reservoirs are referred to as zoonoses or zoonotic diseases. Humans acquire infection from animals either by direct contact, as in the case of pets or farm animals, by ingestion of the animal or inhalation of bacteria in or around its hide, or through an insect vector that transmits the pathogen from the animal to the human via a bite.
Diarrhea caused by Salmonella can occur after handling turtles and contaminating one's hands with their feces, or from ingesting undercooked chicken contaminated with the bacteria, or through other routes such as eating undercooked or raw chicken eggs. The disease tularemia, caused by the organism Francisella tularensis , is often seen in individuals who have recently skinned a rabbit. Similarly, anthrax caused by Bacillus anthracis follows either inhalation of spores from dead animals or hides, or entry of spores into a wound.
In Lyme disease, the deer tick transmits the spirochete Borrelia from the white-footed mouse to the human. Overflow is a phenomenon particularly relevant to zoonotic diseases. Using the example of Lyme disease, the cycle of transmission between tick hosts and animal hosts such as deer and mice leads to the presence of infected ticks that can also infect humans.
Thus the cycle allows the Lyme organisms to overflow from the natural cycle of infection into humans. Reducing the number of infected deer on a New England island through culling, for example, has been shown to greatly decrease the number of infected ticks and almost eliminate infection in humans.
Arthropods reservoirs include insects and arachnids. A vector is commonly understood to be an arthropod that is involved in the transmission of disease. Common insect vectors for bacterial infection include fleas, lice, and flies. Arachnid vectors include mites and ticks. The diseases caused by the bacteria Borrelia which include relapsing fever and the disease referred to in the Unites States as Lyme disease after it was discovered in Lyme, Connecticut infects ticks that take a blood meal from an infected deer or mouse.
These ticks see Figure 3 then inject the bacteria into a human some time later during another blood meal. Other bacterial diseases caused by arthropods include epidemic, murine, and scrub typhus, caused by Rickettsia carried by lice, fleas, and mites, respectively, Rocky Mountain spotted fever also caused by Rickettsia and carried by ticks, and bubonic plague carried by fleas.
Amanda Loffis, Dr. William Nicholson, Dr. Will Reeves, Dr. Chris Paddock. Air can become contaminated by dust or human respiratory secretions containing pathogenic bacteria. Bacteria do not multiply in the air itself, but may be transported by air currents to areas more conducive to their growth.
Infections acquired through the air are characterized as airborne. The classic airborne bacterial infection is tuberculosis. Soil is typically a reservoir for bacteria that form spores when not in a host. The various species of Clostridium can be acquired from exposure of a wound to dirt or soil. These anaerobic bacteria cause tetanus, botulism, and gas gangrene. Anthrax spores can survive for as long as years in soil. Heavy rain, excavation, and tilling may bring them to the surface and cause an outbreak of anthrax among livestock.
In medieval Europe, specific pasturelands were avoided for domesticated animal grazing because of the risk of anthrax. Food, including milk, when not handled properly, can be the reservoir for a wide variety of pathogenic organisms. Food may be contaminated by feces, or the animal itself may be infected, such as in the case of chickens with Campylobacter or Salmonella.
Food can also be contaminated with the ubiquitous spores of Botulinum , which can cause a form of paralysis called botulism. Pasteurization and food sterilization are important public health safeguards against these infections. Food handlers can carry a variety of bacteria on their hands, and indeed there are stringent regulations in many countries regulating food handling and handlers.
Seafood can be contaminated from bacteria in the water. Soft cheeses are common reservoirs for Listeria monocytogenes. Sometimes, unexpected foods become reservoirs for bacterial infection, as in the case of alfalfa and other raw seed sprouts, which since the s were known to be reservoirs for both Salmonella and Escherichia coli. It is thought that the presoaking and germination of the seeds in nutrient solutions is conducive to the growth and multiplication of these pathogenic bacteria.
The seeds themselves can become contaminated at any point in their production and distribution. Transmission via these uncooked foodstuffs has been documented to cause the majority of foodborne bacterial outbreaks in some locations. Water generally becomes a reservoir for infection when it is contaminated by soil microbes, or animal or human feces. Raw sewage may contaminate drinking water during a storm or flood when sewage systems are overwhelmed, or if it is inadequately treated and dumped into local waters see Figure 4.
There is also concern about the potential for terrorists to use water as a reservoir for bioterrorism pathogens. Sources of water contamination. Baltimore: Lippincott Williams and Wilkins. Many inanimate objects are considered fomites, as they are capable of indirectly transmitting infection from one person to another by acting as an intermediate point in the cycle of transmission. Fomites commonly found in households that allow transmission of infection between family members include doorknobs, toilet seats, and utensils.
At daycare centers and pediatrician's offices, infection is transmitted via toys handled by children with contaminated hands. In hospitals, there are countless fomites capable of spreading infection. Many respiratory infections are not spread through aerosols, but rather through respiratory secretions saliva, sputum, etc. You may opt-out of e-mail communications at any time by clicking on the Unsubscribe link in the e-mail. Our Housecall e-newsletter will keep you up-to-date on the latest health information.
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A single copy of these materials may be reprinted for noncommercial personal use only. This content does not have an English version. This content does not have an Arabic version. See more conditions. Request Appointment. Bacterial vs. Products and services. What's the difference between a bacterial infection and a viral infection? Answer From Pritish K. Tosh, M. Thank you for Subscribing Our Housecall e-newsletter will keep you up-to-date on the latest health information.
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