Pathogenic Bacteria

Phylum Proteobacteria
This phylum is the largest taxonomic group and includes the majority of the chemoheterotrophic gram negative species.  They vary greatly in their environment, metabolism, and functions.

Class Gammaproteobacteria
Of the proteobacteria, this class contains the most bacteria which vary greatly in their physiological makeup.   
Vibrio cholerae
V. cholerae is a gram negative, curved, motile bacteria that causes one of the most rapidly fatal diseases known, cholera.  While its natural environment is within marine shellfish, humans become infected with this pathogen when consuming water that is contaminated with human feces.  Once inside a human host, it adheres to and colonizes the small intestines.  Here, it secretes the cholera toxin which acts on the intestinal mucosal cells to produces huge amounts of cyclic AMP, causing a net flow of chloride ions and water out of the tissues.  As a result, a person will experience extreme abdominal cramps, massive diarrhea, or characteristically, rice water stool, and thus, severe dehydration occurs.  If fluids lost are not replaced, the person's skin will become cyanotic as their blood thickens, their circulatory system will collapse, and they will go into shock, leading to death. A person who becomes infected with cholera does not die because their body's immune system cannot fight off the infection, but because the symptoms of infection occur so rapidly that the body does not have enough time to rid itself of the bacteria.  Those that survive the disease may become asymptomatic carriers. 

Yersinia pestis
Y. Pestis is commonly known as the Plague or Black Death and killed over 100 million Europeans in the 6th century.  While it is primarily a rodent pathogen, fleas that feed on infected rats have the ability to transmit the bacteria to humans.  The bacteria inside the infected flea proliferate and clump within the the gut, starving the flea.  In order for the flea to feed, it must regurgitate the clumped bacteria, and thus injecting the bacteria into the blood stream of its next meal.  When Y. pestis enters a human host, it rapidly reproduces within the host lymph nodes, initiating a hemorrhagic immune response, causing them to become hot and very swollen; hence the term black buboes.  As the bacteria circulate in the the blood they enter the liver and spleen where the infected host can go into endotoxic shock.  Within the blood, disseminating intravascular coagulation occurs in which blood clots form and thus the skin of the extremities become black.  Upon entering the lungs, Pneumonic Plague ensues and Y. pestis becomes infectious to others through airborne transmission.  This form of the disease is extremely virulent and causes necrotizing pneumonia.  If untreated, there is a 100% mortality rate for Pneumonic Plague as the lung tissue liquifies and cavities within the tissue form.  Its victims eventually die from Acute Respiratory Distress Syndrome (ARDS).

Phylum Actinobacteria (The High G+C Gram Positives)
This group exhibits a large degree of pleomorphism.  Many are filamentous with numerous branches and resemble filamentous fungi in their morphology.

Corynebacterium diphtheriae
This bacteria is a gram positive, club-shaped, non-motile bacteria.  It is the infective agent of Diphtheria, a highly fatal disease in which one in every ten infected died before the DTaP vaccine was created.  C. diptheriae enters the body when it is inhaled from aerosols in the air where it colonizes the throat and produces the dipthheria toxin.  This toxin causes necrotic injury to the respiratory epithelial.  The toxin can then enter the blood where it targets the heart, peripheral nerves, and other organs.  There it inactivates translation and kills host cells.  As it colonizes the throat, it creates a fibrin pseudomembrane.  This thick, gray membrane causes the neck to swell, making it difficult to breath and swallow, a diagnostic characteristic of the disease.  The membrane begins in the throat and continues along into the lungs.  Those that become infected with this disease primarily die from respiratory problems and heart failure.

Mycobacterium tuberculosis
M. tuberculosis is the infective agent of Tuberculosis or "consumption" as it is nick-named.  It is a gram positive, non-motile bacteria and is the leading cause of death world-wide from a single infectious disease.  Lipids in its cell wall, such as mycolic acid, chord factor, and wax-D, make it highly resistant to antibiotics, complement deposition, and phagocytic killing.  The infection begins when the bacteria, carried in air droplets, are inhaled into the lungs.  Inside the lungs they enter the alveoli where they invade and inactivate alveolar macrophages.  Cytokine release recruits macrophages and T cells which in turn activate more macrophages to the infection site.  Phagocytes that are unable to destroy bacteria cause considerable damage and produce lesions within the lung tissues.  As the host's immune response attempts to rid itself of the infection, a tubercule forms from deposited fibrin which becomes calcified and serves as a barrier to the lesion.  Eventually, the lesion necrotizes and liquifies, causing the infected person to become more contagious and allowing the bacteria to spread rapidly to other areas of the body.  Systemic spread of the bacteria is 100% fatal if it is not treated.

Phylum Firmicutes (The Low G+C Gram Positives)
In addition to gram positive bacteria, this group also contains gram negative bacteria.  Important endospore-forming, lactic acid producing, and cell wall lacking species are included in this phylum.

Streptococcus pneumoniae
S. pneumoniae or pneumococcus is a gram positive, non-motile, diplococci bacteria.  It is significant in that it is the most common cause of bacterial pneumonia and otitis media, as well as the leading cause of infant meningitis in the United States.  Additionally, it kills more in the United States than all other vaccine preventable bacteria combined.  While it is a normal inhabitant of the upper respiratory tract, it usually becomes pathogenic only among children, the elderly, and immunocompromised individuals.  They travel in the air via aerosols and enter the nasopharynx upon inhalation.  If the bacteria is able to get passed the ciliated cells, it can then colonize the respiratory tract where is has access to the Eustachian tube of the inner ear as well as the lungs.  When an infection of the lungs occurs, the bacterial capsule prevents opsonization and their cell wall fragments trigger a local inflammatory response causing major damage to the lung tissues.  Inflammation produces pus which reduces the gas exchange in the alveoli causing the person to become cyanotic due to the lack of oxygen able to be taken up by the body.  If an ear infection caused by S. pneumoniae occurs, the bacteria can spread to the blood causing a condition known as bacteremia.  Here, the bacteria can cross the blood brain barrier, causing meningitis, and septic shock.