The most common pathogenic bacteria can vary by community or population, or if considering a particular type of infection. The most common strains also vary by time period based on effective or new treatments and preventative measures. In today’s world, antibiotic resistant bacteria are becoming more common, with many of the common pathogenic bacteria having resistant strains.
Staphylococcus species are extremely common bacteria in the human intestinal system, and even on the skin of up to a third of adults. The presence of the bacteria on the skin allows easy access to open wounds, resulting in the dangerous skin infections known as staph. The Centers for Disease Control surveillance network has found that Staphylococcus aureus is the most common cause of pneumonia caught in hospital settings, the most common cause of infection in surgical wounds, and the second most common cause of nosocomial (i.e. infected at the hospital) bloodstream infections. Another major cause of nosocomial infection is Staphylococcus epidermidis, which infiltrates the body via medical devices.
Scanning electron microscope image of Staphylococcus aureus, CDC
According to 2011 research out of Northern Arizona University and Translational Genomics Research Institute in Flagstaff, not only is this species of Staphylococcus the most common cause of clinical infections, it is also a common contaminant of meat in the food supply. Multidrug resistant strains are also becoming more and more common among these cases of contamination. One form of antibiotic-resistant S. aureus is MRSA (pronounced “mer-suh”, methicillin-resistant Staphylococcus aureus). This strain accounted for more than half of hospital-associated S. aureus infections by 2007 based on the MRSA Prevalence Study.
Respiratory tract infection
The most common site of pathogenic invasion is considered to be the respiratory tract. Though the main pathogens are viruses, bacteria are responsible for the development of secondary infections, such as pneumonia, as well as some primary infections. The most common cause of lower respiratory tract infection in adults is Streptococcus pneumoniae. In children, the common cause of airway infection is Haemophilus influenzae type b.
Scanning electron microscope image of Streptococcus pneumoniae, CDC
The most common cause of food-related illness is bacteria. These pathogenic bacteria tend to cause gastrointestinal disorders, such as diarrhea and vomiting, while putting the person at risk for dehydration. The most common of these pathogenic bacteria varies by region of the world, but topping the list provided by the World Health Organization are Salmonella and Campylobacter. In addition, in developing countries, the organism that causes cholera, Vibrio cholerae, is still a major contaminant of the water supply and commonly causes disease.
Scanning electron microscope image of Vibrio cholerae, Dartmouth
A list of other bacteria that are commonly found on or in the human body, though not necessarily causing disease, is available from the OnlineTextbook of Bacteriology.