Bacterial infections occur when single-celled micro-organisms called bacteria colonize the human body. While this colonization is usually fended off by the immune system, bacteria cause disease when they cannot be controlled and begin to interfere with normal function. Bacteria can invade almost any system of the body, and do so through many routes of entry. Some types of bacteria normally live symbiotically with humans, especially on the skin and in the gut, but these can cause disease when they grow out of control or enter parts of the body where they are not normally found. There are millions of different species of bacteria, but only a handful account for the majority of infections in humans. It is difficult to quantify exactly how much disease is caused by bacterial infections, but it is significant. In 2002, lower respiratory infections and diarrheal diseases, which are usually caused by bacteria, were the two leading causes of death in children under 5 years, accounting for 17.7 percent and 14.9 percent of all deaths before age 5, respectively. For adults, tuberculosis alone was the third leading cause of death for those ages 15–59, accounting for 1,036,000 deaths in 2002. These numbers give an indication of the devastating impact that bacterial infections have on human health.
Some bacteria tend to infect certain populations, such as children, the elderly, or people with compromised immune systems. This last group has grown recently with the emergence of the HIV/AIDS epidemic, and subsequently many types of bacterial infections are reemerging as major threats to public health. The most striking example of this is tuberculosis, which was thought to be well controlled in the 1970s, but has now resurfaced, especially in areas where AIDS is prevalent. In addition to these challenges, many bacteria are quickly developing resistance to the drugs currently used to treat them. Bacterial infections are a critical challenge in current global health, especially in the context of the AIDS pandemic and emerging drug resistance.
Respiratory infections can be divided into two categories: upper respiratory infections (URIs) and lower respiratory infections (LRIs). Upper respiratory infections involve the nasal cavities, sinuses, pharynx, and larynx, and are most commonly caused by viruses. However, some URIs are caused by bacteria, including group A beta-hemolytic Streptococcus, which causes the syndrome known as strep throat. Other bacterial causes of URIs include Streptococcus pneumoniae and Haemophilus influenzae type B, although the latter has been largely eliminated in nations with vaccination programs. Symptoms of URIs include runny nose, cough, sore throat, fever, and sneezing. Otitis media, infection of the middle ear, is also commonly associated with URIs because the middle ear and the nasal cavity are anatomically connected by the Eustachian tube.
Lower respiratory infections involve the lungs and bronchi, and lead to the most mortality of all infectious diseases, causing around 6.8 percent of all deaths worldwide, and one in three newborn deaths. Often used synonymously with pneumonia, LRIs are much more commonly caused by bacteria than URIs. Common bacterial pathogens include Streptococcus pneumoniae and Staphylococcus aureus, which are both gram-positive bacteria. Many strains of gram-negative bacteria also cause pneumonia; these tend to originate in the gastrointestinal tract and get into the lungs when vomit is aspirated. LRIs are characterized by high fever, shortness of breath, chest pain, and productive cough with yellow-green sputum and blood. The alveoli of the lungs become filled with fluid, compromising the exchange of oxygen for carbon dioxide. When severe, this can be fatal. LRIs can also lead to acute respiratory distress syndrome (ARDS), which is an inflammatory response in the lungs leading to rapid filling of the alveoli with fluid, respiratory failure, and death. Bacterial infections in the lungs can also lead to abscesses or collections of fluid in the space surrounding the lungs called pleural effusions.
Diarrheal diseases are among the most common disease types in the world, and the causes are numerous. Bacterial infections of the intestines are a major cause of diarrhea, and impart a sizable disease burden on human populations, especially children, and especially in poor countries. Diarrheal diseases account for approximately 18 percent of deaths in children under 5 around the world, with 40 percent of these deaths occurring in Africa. Diarrhea consists of frequent, loose, watery stools, which may contain blood or fat. It is sometimes accompanied by abdominal pain or cramping, vomiting, or fever. Because so much fluid is being lost from the gastrointestinal system, diarrhea may precipitate severe dehydration, which is the primary cause of death from diarrheal illnesses. The most common bacterial causes of diarrhea are E. coli O157:H7, Salmonella enteridis, Vibrio cholerae, and Shigella.
Bacterial meningitis is inflammation of the meninges due to bacterial infection. Symptoms include sensitivity to light, stiff neck, headache, fever, and confusion, although usually only one or a few symptoms are present. The symptoms in small children can be more difficult to detect, and may consist only of inactivity, poor feeding, and irritability. It is important to diagnose bacterial meningitis early and aggressively treat it because it can progress rapidly, causing death in a matter of hours, and is usually curable with antibiotics. The three most common bacteria that cause meningitis are Haemophilus influenzae type b, Neisseria meningitidis, and Streptococcus pneumoniae, although with the advent of the H. influenzae vaccine, incidence of meningitis caused by that strain has decreased precipitously. The bacteria are spread by contact with nose or throat mucus from an infected person, so close contacts are usually given antibiotics prophylactically.
Endocarditis is inflammation of the inner layer of the heart, especially the heart valves. Bacteria can colonize the valves when they are damaged and blood flow around them is altered. Endocarditis can lead to spiking fever, bacteria in the bloodstream, emboli, and heart murmur. Many species of bacteria cause endocarditis, the most common of which are Staphylococcus aureus, alpha hemolytic Streptococci, and Enterococci. It is treated with high-dose antibiotics or surgical removal of the infected valve.
Cellulitis is an infection of the tissue underlying the skin, characterized by inflammation and pain. It generally occurs when bacteria that normally live on the surface are introduced into the tissue below through a break in the skin. This can be done by cuts, scratches, burns, during surgery, or by placement of a catheter into a vein. Under the skin, the bacteria can proliferate, damage, and kill the host cells. There are many types of bacteria that can cause cellulitis, the most common being Staphylococcus aureus and Group A Streptococcus. These infections are usually easily treatable with antibiotics, but can persist, spread to other organs, and cause significant damage if left untreated. One well-known complication of cellulitis is necrotizing fasciitis, which infects the fascia just deep to the subcutaneous tissues. This infection spreads quickly and is fatal if not treated.
Sexually transmitted infections (STIs) comprise another group of bacterial infections, the most common of which are gonorrhea, chlamydia, and syphilis. STIs are very common around the world, and often go untreated due to lack of access to healthcare and stigma surrounding sex-related issues. This poses an important challenge, because having one STI with active symptoms predisposes one to being infected with another. In the context of the HIV/AIDS pandemic, this is a cause for concern, because being infected with an STI increases one’s risk of being infected by HIV. Symptoms of STIs include sores on the genitals, purulent discharge, and painful urination, depending on the pathogen. In women, they can also induce pelvic inflammatory disease, ectopic pregnancies, and miscarriages, and in poor regions of the world, STIs and their complications are the most common cause of illness and death for women. STIs can also cause infertility in men, and be passed on to babies born to infected mothers, potentially resulting in blindness, disability, or death. In addition, STIs are often asymptomatic, and infected individuals often do not know that they are infected unless they are tested, yet can still infect others.
Urinary tract infections include infections of the urethra (urethritis), the bladder (cystitis), and the ureters and kidneys (pyelonephritis). Symptoms include frequent urination, pain in the bladder, cloudy urine, fever, and nausea. The most common infectious agents are Escherichia coli and Staphylococcus saprophyticus, which are easily treated with oral antibiotics.
However, if the infection ascends to the kidneys, indicated by tenderness in the ribs next to spine, intravenous antibiotics may be necessary. Untreated pyelonephritis can result in kidney failure, especially in children. Urinary tract infections are most common in sexually active women and individuals with urinary catheters.
All of the different types of infections discussed thus far can potentially lead to bacteremia, or bacteria in the bloodstream. Transient bacteremia happens frequently. For example, after brushing one’s teeth, it is normal to have minute amounts of bacteria in the bloodstream. These bacteria are usually cleared by the immune system and do not lead to disease.
However, when a bacterial infection is present, much higher levels of bacteria can be released into the blood stream over a longer period of time. In these cases, the immune system may react with a systemic inflammatory response called sepsis. This response is an overreaction that causes inflammation and coagulation throughout the whole body. This can progress to multiorgan dysfunction syndrome and death. Sepsis is characterized by rapid heart rate, high fever, hyperventilation, and high or low white blood cell count. Management and treatment consists of antibiotics, drainage of fluid collections, fluid replacement, and organ support.
Clearly, bacterial infections are diverse in their presentation and severity. It is striking, however, that on the global scale such a large proportion of morbidity and mortality is accounted for by a relatively small cohort of pathogens. Bacteria are a major health concern in both rich and poor countries. While some infections like cholera are more of a concern in areas with poor sanitation, others such as methicilline-resistant Staphylococcus aureus are a growing concern in hospitals of industrialized countries.
One example of a pathogen with a truly global reach is Mycobacterium tuberculosis. This bacterium causes tuberculosis (TB), which is considered a reemerging infectious disease, largely due to coinfection with HIV. About one-third of the world population is infected with tuberculosis, meaning that they have the bacteria in their lungs; however, only 10 percent of those will develop active disease. The other 90 percent remain asymptomatic, with the bacteria contained in their lungs by their immune systems. However, with the spread of HIV and the epidemic weakening of immune systems, more people are unable to keep their chronic infections at bay, and progress to develop TB.
Tuberculosis illustrates another important aspect of bacterial infections: drug resistance. TB is curable with the administration of triple antibiotic therapy. Yet this therapy needs to be administered rigorously for several months in order to work. If the drug regimen is not followed well, there is a risk that drug resistance will develop. Drug resistance follows the basic principles of evolution. When there is a population of bacteria in an infection, and they are treated with an antibiotic, some of them may develop mutations that make them unsusceptible to the drug. With the fast pace at which most bacteria reproduce and the large numbers of organisms in a given infection, this is almost inevitable. If those resistant bacteria survive beyond the course of treatment, they may reproduce and become an infection of drug-resistant bacteria. This is what has been happening with TB; the improper administration of drug therapy over time has led to strains of M. tuberculosis that are resistant to the standard treatment regimens. In some parts of the world, there are cases of multidrug-resistant tuberculosis (MDR-TB) that are essentially untreatable.
Bacteriology; Chlamydia Infections; Cholera; E. Coli Infections; Gonorrhea; Meningitis; Pneumonia; Salmonella Infections; Staphylococcal Infections; Streptococcal Infections; Tuberculosis.
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