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El Streptoccoccus mutans (figura de arriba)  siempre se encuentra asociado al inicio de la caries por su capacidad de colonizar la superficie del esmalte dental en presencia de azúcar, que da lugar a la producción de ácidos que desmineralizan la superficie del esmalte. Esta desmineralización inicia un proceso conocido como “mancha blanca”, que si no se revierte termina en la formación decaries.

estreptococus sanguis
(2)  Many of the normal flora are either pathogens or opportunistic pathogens, The asterisks indicate members of the normal flora a that may be considered major pathogens of humans.

S. aureus. Gram stain.
(3) Streptococcus mutans is the primary bacterium involved in plaque formation and initiation of dental caries.  Viewed as an opportunistic infection,dental disease is one of the most prevalent and costly infectious diseases in the United States.

Streptococcus mutans. Gram stain. CDC
(4) Enterococcus faecalis was formerly classified as Streptococcus faecalis. The bacterium is such a regular a component of the intestinal flora, that many European countries use it as the standard indicator of fecal pollution, in the same way we use E. coli inthe U.S.  In recent years, Enterococcus faecalis has emerged as a significant, antibiotic-resistant, nosocomial pathogen.

Vancomycin Resistant Enterococcus faecalis. Scanning E.M. CDC
(5) Streptococcus pneumoniae is present in the upper respiratory tract of about half the population.  If it invades the lower respiratory tract it can cause pneumonia.  Streptococcus pneumoniae causes 95percent of all bacterial pneumonia.

Streptococcus pneumoniae. Direct fluorescent antibody stain. CDC.
(6) Streptococcus pyogenes refers to the Group A, Beta-hemolytic streptococci. Streptococci cause tonsillitis (strep throat), pneumonia, endocarditis. Some streptococcal diseases can lead to rheumatic fever or nephritis which can damage the heart and kidney.

Streptococcus pyogenes. Gram stain.(7) Neisseria and other Gram-negative cocci are frequent inhabitants of the upper respiratory tract, mainly the pharynx. Neisseria meningitidis, an important cause of bacterial meningitis, can colonize as well, until the host can develop active immunity against the pathogen.

Neisseria meningitidis. Gram stain.
(8) While E. coli is a consistent resident of the small intestine, many otherenteric bacteria may reside here as well, including Klebsiella, Enterobacter and Citrobacter.  Some strains of E. coli are pathogens that cause intestinal infections, urinary tract infections and neonatal meningitis.

E. coli. Scanning E.M. Shirley Owens. Center for Electron Optics. Michigan State University.
(9) Pseudomonas aeruginosa is the quintessential opportunistic pathogen of humansthat can invade virtually any tissue.  It is a leading cause of hospital-acquired (nosocomial) Gram-negative infections, but its source is often exogenous (from outside the host).

Colonies of Pseudomonas aeruginosa growing on an agar plate. The most virulent Pseudomonas species produce mucoid colonies and green pigments such as this isolate.
(10) Haemophilus influenzae is a frequent secondaryinvader to viral influenza, and was named accordingly.  The bacterium was the leading cause of meningitis in infants and children until the recent development of the Hflu type B vaccine.

Haemophilus influenzae. Gram stain.
(11) The greatest number of bacteria are found in the lower intestinal tract, specifically the colon and the most prevalent bacteria are the Bacteroides, a group ofGram-negative, anaerobic, non-sporeforming bacteria.  They have been implicated in the initiation colitis and colon cancer.

Bacteroides fragilis. Gram stain.
(12) Bifidobacteria are Gram-positive, non-sporeforming, lactic acid bacteria. They have been described as "friendly" bacteria in the intestine of humans. Bifidobacterium bifidum is the predominant bacterial species in the intestine of...