S. aureus can cause a range of illnesses, from minor skin infections, such as pimples, impetigo, boils (furuncles), cellulitis folliculitis, carbuncles, scalded skin syndrome, and abscesses, to life-threatening diseases such as pneumonia, meningitis, osteomyelitis, endocarditis, toxic shock syndrome (TSS), bacteremia, and sepsis.
Staphylococcus aureus is a bacterium that is a member of the Firmicutes, and is frequently found in the human respiratory tract and on the skin. Although S. aureus is not always pathogenic, it is a common cause of skin infections (e.g. boils), respiratory disease (e.g. sinusitis), and food poisoning. Disease-associated strains often promote infections by producing potent protein toxins, and expressing cell-surface proteins that bind and inactivate antibodies. The emergence of antibiotic-resistant forms of pathogenic S. aureus (e.g. MRSA) is a worldwide problem in clinical medicine.
Staphylococcus was first identified in 1880 in Aberdeen, United Kingdom, by the surgeon Sir Alexander Ogston in pus from a surgical abscess in a knee joint. This name was later appended to Staphylococcus aureus by Rosenbach who was credited by the official system of nomenclature at the time.
It is estimated that 20% of the human population are long-term carriers of S. aureus which can be found as part of the normal skin flora and in anterior nares of the nasal passages. S. aureus is the most common species of staphylococcus to cause Staph infections and is a successful pathogen due to a combination of nasal carriage and bacterial immuno-evasive strategies.
S. aureus can cause a range of illnesses, from minor skin infections, such as pimples, impetigo, boils (furuncles), cellulitis folliculitis, carbuncles, scalded skin syndrome, and abscesses, to life-threatening diseases such as pneumonia, meningitis, osteomyelitis, endocarditis, toxic shock syndrome (TSS), bacteremia, and sepsis. Its incidence ranges from skin, soft tissue, respiratory, bone, joint, endovascular to wound infections. It is still one of the five most common causes of nosocomial infections and is often the cause of postsurgical wound infections. Each year, some 500,000 patients in American hospitals contract a staphylococcal infection.
Role in disease
S. aureus is responsible for many infections but it may also occur as a commensal. The presence of S. aureus does not always indicate infection. S. aureus can survive from hours to weeks, or even months, on dry environmental surfaces, depending on strain.
S. aureus can infect tissues when the skin or mucosal barriers have been breached. This can lead to many different types of infections including furuncles and carbuncles (a collection of furuncles).
S. aureus infections can spread through contact with pus from an infected wound, skin-to-skin contact with an infected person by producing hyaluronidase that destroys tissues, and contact with objects such as towels, sheets, clothing, or athletic equipment used by an infected person. Deeply penetrating S. aureus infections can be severe. Prosthetic joints put a person at particular risk of septic arthritis, and staphylococcal endocarditis (infection of the heart valves) and pneumonia. Strains of S. aureus can host phages, such as Φ-PVL (produces Panton-Valentine leukocidin), that increase virulence.
S. aureus is extremely prevalent in atopic dermatitis patients. It is mostly found in fertile, active places, including the armpits, hair, and scalp. Large pimples that appear in those areas may exacerbate the infection if lacerated. This can lead to staphylococcal scalded skin syndrome (SSSS). A severe form of this, Ritter's disease, can be observed in neonates. Animal infections
S. aureus can survive on dogs, cats, and horses, and can cause bumblefoot in chickens. Some believe health-care workers' dogs should be considered a significant source of antibiotic-resistant S. aureus, especially in times of outbreak. S. aureus is one of the causal agents of mastitis in dairy cows. Its large polysaccharide capsule protects the organism from recognition by the cow's immune defenses.
Salmonella infections are zoonotic and can be transferred between humans and nonhuman animals
Salmonella is a genus of rod-shaped, Gram-negative, non-spore-forming, predominantly motile enterobacteria with diameters around 0.8 to 1.5 µm, lengths from 2 to 5 µm, and flagella that grade in all directions (i.e., peritrichous). They are chemoorganotrophs, obtaining their energy from oxidation and reduction reactions using organic sources, and are facultative anaerobes. There are only two species of Salmonella; Salmonella bongori and Salmonella enterica of which there are innumerable subspecies. Most subspecies produce hydrogen sulfide, which can readily be detected by growing them on media containing ferrous sulfate, such as TSI. Most isolates exist in two phases: a motile phase I and a nonmotile phase II. Cultures that are nonmotile upon primary culture may be switched to the motile phase using a Cragie tube.
Salmonella is closely related to the Escherichia genus and are found worldwide in cold- and warm-blooded animals (including humans), and in the environment. They cause illnesses such as typhoid fever, paratyphoid fever, and foodborne illness.
Salmonella as disease-causing agents
Salmonella infections are zoonotic and can be transferred between humans and nonhuman animals. Many infections are due to ingestion of contaminated food. For example, recent FDA studies link Guatemalan cantaloupes with Salmonella panama. In speaking of other salmonella serotypes, enteritis Salmonella and Salmonella typhoid/paratyphoid Salmonella, the latter—because of a special virulence factor and a capsule protein (virulence antigen)—can cause serious illness, such as Salmonella enterica subsp. enterica serovar Typhi. Salmonella typhi is adapted to humans and does not occur in other animals.
Salmonella species are facultative intracellular pathogens that enter cells via macropinosomes.
Enteritis salmonellosis or food poisoning Salmonella
This is a group consisting of potentially every other serotype (over a thousand) of the Salmonella bacteria, most of which have never been found in humans. These are encountered in various Salmonella species, most having never been linked to a specific host, but can also infect humans. It is therefore a zoonotic disease.
The organism enters through the digestive tract and must be ingested in large numbers to cause disease in healthy adults. Gastric acidity is responsible for the destruction of the majority of ingested bacteria. Bacterial colonies may become trapped in mucus produced in the oesophagus.
Salmonellosis is a disease caused by raw or undercooked food. Infection usually occurs when a person ingests foods that contain a high concentration of the bacteria, similar to a culture medium. In otherwise healthy adults, the symptoms are usually mild. Normally, no sepsis occurs, but it can occur exceptionally as a complication in elderly or weakened patients (e.g., those with Hodgkin's disease).
However, infants and young children are much more susceptible to infection, easily achieved by ingesting a small number of bacteria. In infants, contamination through inhalation of bacteria-laden dust is possible. After a short incubation period of a few hours to one day, the bacteria multiply in the intestinal lumen, causing an intestinal inflammation with diarrhea that is often mucopurulent (containing mucus or pus) and bloody. In infants, dehydration can cause a state of severe toxicosis. Extraintestinal localizations are possible, especially Salmonella meningitis in children, osteitis, etc.
Enteritis Salmonella (e.g., Salmonella enterica subsp. enterica serovar enteritidis) can cause diarrhea, which usually does not require antibiotic treatment. However, in people at risk such as infants, small children, the elderly, Salmonella infections can become very serious, leading to complications. If these are not treated, HIV patients and those with suppressed immunity can become seriously ill. Children with sickle cell anaemia who are infected with Salmonella may develop osteomyelitis. Treatment of osteomyelitis, in this case, will be to use fluoroquinolones (Ciproflaxacin, Levofloxacin, etc. and Nalidixic acid).
Salmonella bacteria can survive for weeks outside a living body, and they are not destroyed by freezing. Ultraviolet radiation and heat accelerate their demise; they perish after being heated to 55 °C (131 °F) for 90 min, or to 60 °C (140 °F) for 12 min. To protect against Salmonella infection, heating food for at least ten minutes at 75 °C (167 °F) is recommended, so the centre of the food reaches this temperature.
Most people with salmonellosis develop diarrhea, fever, vomiting, and abdominal cramps 12 to 72 hours after infection. In most cases, the illness lasts four to seven days, and most people recover without treatment. In some cases, though, the diarrhea may be so severe, the patient becomes dangerously dehydrated and must be taken to a hospital. At the hospital, the patient may receive intravenous fluids to treat the dehydration, and may be given medications to provide symptomatic relief, such as fever reduction. In severe cases, the Salmonella infection may spread from the intestines to the blood stream, and then to other body sites, and can cause death, unless the person is treated promptly with antibiotics. The elderly, infants, and those with impaired immune systems are more likely to develop severe illness.
An infectious process can only begin after living salmonellae (not only their toxins) reach the gastrointestinal tract. Some of the microorganisms are killed in the stomach, while the surviving salmonellae enter the small intestine and multiply in tissues (localized form). By the end of the incubation period, the macro-organisms are poisoned by endotoxins released from the dead salmonellae. The local response to the endotoxins is enteritis and gastrointestinal disorder. In the generalized form of the disease, salmonellae pass through the lymphatic system of the intestine into the blood of the patients (typhoid form) and are carried to various organs (liver, spleen, kidneys) to form secondary foci (septic form). Endotoxins first act on the vascular and nervous apparatus, manifested by increased permeability and decreased tone of the vessels, upset thermal regulation, vomiting and diarrhea. In severe forms of the disease, enough liquid and electrolytes are lost to upset the water-salt metabolism, to decrease the circulating blood volume and arterial pressure, and to cause hypovolemic shock. Septic shock may develop. Shock of mixed character (with signs of both hypovolemic and septic shock) are more common in severe salmonellosis. Oliguria and azotemia develop in severe cases as a result of renal involvement due to hypoxia and toxemia.
A small number of people afflicted with salmonellosis experience reactive arthritis, which can last months or years and can lead to chronic arthritis.
In Germany, food poisoning infections must be reported. Between 1990 and 2005, the number of officially recorded cases decreased from approximately 200,000 to approximately 50,000 cases. In the USA, about 40,000 cases of Salmonella infection are reported each year. According to the World Health Organization, over 16 million people worldwide are infected with typhoid fever each year, with 500,000 to 600,000 fatal cases.
The AvrA toxin injected by the type three secretion system of Salmonella Typhimurium works to inhibit the innate immune system by virtue of its serine/threonine acetyltransferase activity, and requires binding to eukaryotic target cell phytic acid (IP6). This leaves the host more susceptible to infection. In a 2011 paper, Yale University School of Medicine researchers described in detail how Salmonella is able to make these proteins line up in just the right sequence to invade host cells. "These mechanisms present us with novel targets that might form the basis for the development of an entirely new class of antimicrobials," said Professor Dr. Jorge Galan, senior author of the paper and the Lucille P. Markey Professor of Microbial Pathogenesis and chair of the Section of Microbial Pathogenesis at Yale. In the new National Institutes of Health-funded study, Galan and colleagues identify what they call a bacterial sorting platform, which attracts needed proteins and lines them up in a specific order. If the proteins do not line up properly, Salmonella, as well as many other bacterial pathogens, cannot "inject" them into host cells to commandeer host cell functions, the lab has found. Understanding how this machine works raises the possibility of new therapies that disable this protein delivery machine, thwarting the ability of the bacterium to become pathogenic. The process would not kill the bacteria as most antibiotics do, but would cripple its ability to do harm. In theory, this means bacteria such as Salmonella might not develop resistance to new therapies as quickly as they usually do to conventional antibiotics.
Escherichia coli (commonly abbreviated E. coli) is a Gram-negative, facultative anaerobic, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms (endotherms). Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls due to food contamination. The harmless strains are part of the normal flora of the gut, and can benefit their hosts by producing vitamin K2, and by preventing the establishment of pathogenic bacteria within the intestine.
E. coli and related bacteria constitute about 0.1% of gut flora, and fecal–oral transmission is the major route through which pathogenic strains of the bacterium cause disease. Cells are able to survive outside the body for a limited amount of time, which makes them ideal indicator organisms to test environmental samples for fecal contamination. There is, however, a growing body of research that has examined environmentally persistent E. coli which can survive for extended periods outside of the host.
The bacterium can be grown easily and inexpensively in a laboratory setting, and has been intensively investigated for over 60 years. E. coli is the most widely studied prokaryotic model organism, and an important species in the fields of biotechnology and microbiology, where it has served as the host organism for the majority of work with recombinant DNA.