Bacterial infections can be transmitted by a variety of mechanisms. In order to be spread, a sufficient number of organisms must survive in the environment and reach a susceptible host. Many bacteria have adapted to survive in water, soil, food, and elsewhere. Some infect vectors such as animals or insects before being transmitted to another human. New species and new variants of familiar species continue to be discovered, particularly as we intrude into new ecosystems.
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. There are also microbes called protozoa. What is a microbe easy definition? A microorganism or microbe is an organism which is microscopic, which means so small that people cannot see them with naked eye.
The study of microorganisms is called microbiology. Microorganisms include bacteria, fungi, archaea, protists and viruses, and are among the earliest known life forms. What is the smallest microorganism? What is the importance of microorganisms? Microbes are important in human culture and health in many ways, serving to ferment foods, treat sewage, produce fuel, enzymes and other bioactive compounds.
They are essential tools in biology as model organisms and have been put to use in biological warfare and bioterrorism. What are the 4 main microorganisms? The major groups of microorganisms—namely bacteria, archaea, fungi yeasts and molds , algae, protozoa, and viruses—are summarized below. Do viruses have a nucleus? While there some advanced viruses that seem fancy, viruses don't have any of the parts you would normally think of when you think of a cell. They have no nuclei, mitochondria, or ribosomes.
Some viruses do not even have cytoplasm. The capsid protects the core but also helps the virus infect new cells. Sometimes, the infection will not go away and your doctor may have to try a different type of antibiotic.
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Get Resources. Donate Button — Arrow. Bacterial Infections Bacteria are microscopic single-cell microorganisms that are found all around us. Examples of bacterial infections Bacteria must enter your body for them to cause an infection.
What are the symptoms of a bacterial infection? If you have bacterial pneumonia, you may experience Fever Cough, with phlegm Shortness of breath Sweating Shaking chills Headache Muscle pain Fatigue Chest pain with breathing If you have a urinary tract infection, you may have some of these symptoms: Sudden and extreme urges to void pass urine Frequent urges to void Burning, irritation or pain as you void A feeling of not emptying your bladder completely A feeling of pressure in your abdomen or lower back Thick or cloudy urine — it may contain blood Fever The common element with most bacterial infections are: Fever Chills Pain or discomfort in the affected area But if the infection is in a joint, that joint and the surrounding area will likely hurt; if you have a sinus infection, you will probably have a headache and foul nasal discharge, and so on.
You can get these infections by consuming things like:. Some infections are spread to people from an infected animal. One example is the rabies virus, which you can get if an infected animal bites you. Another example is toxoplasmosis. There are many different types of biting bugs, including ticks, mosquitoes, and lice. In some cases, you can get an infection if a bug carrying around an infectious microorganism bites you.
Some examples include malaria, Lyme disease, and West Nile Virus. Not all infections are spread in the same way. While one infection may be transmitted via infected blood, another may be transmitted by the bite of an insect. Some infections have very characteristic symptoms. Your doctor may be able to make a diagnosis based off of these symptoms, your medical history, and a physical examination.
In other cases, it can be hard to determine what type of organism may be causing your condition. For example, some bacterial and viral infections can have very similar symptoms. Where this sample is collected from depends on your illness and the type of organism suspected. Some sample types can include:.
In some cases, they may also want to take a biopsy of the affected tissue to examine it. There are many actions that you can take to prevent the spread of infections. Be sure to follow the tips below:. Infections can be caused by a variety of different organisms, including viruses, bacteria, fungi, and parasites. The different ways that you can get an infection can be just as diverse as the organisms that cause them.
You may need additional medications to treat your condition. Bacterial infections can affect wounds, burns, and organs within the body. They occur when bacteria enter the body that shouldn't. Signs of infection…. Bacterial and viral infections are often transmitted in similar ways, but symptoms and treatment methods may vary depending on the cause of your….
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