The domain bacteria are separated into 5 different groups:

1. proteobacteria
   
2. chlamydias
   
3. spirochetes
   
4. cyanobacteria
   
5. gram-positive bacteria

1) PROTEOBACTERIA

​Proteobacteria is phylum gets its name from the word "proteo", which means “first”.  The proteobacteria phylum is the largest and most diverse bacterial phylum. Many of the common gram-negative bacteria that microbiologists and medical professionals are familiar with belong to this phylum.

PROTEABACTERIA ARE DIVIDED INTO 5 CLASSES

1)  Alpha proteobacteria
2)  Beta proteobacteria
3)  Gamma proteobacteria
4)  Delta proteobacteria
5)  Epsilon proteobacteria 

Rickettsiae

E. Coli

 Alpha Proteobacteria

Title: Pathology: Histology: Pap Smear Description: Human pap smear showing clamydia in the vacuoles at 500x and stained with H&E.

     Alphaproteabacteria are a class of gram-negative proteobacteria that are oligotrophic, which means that they can survive in habitats that provide little nutrients. Alphaproteobacteria live in extreme environments such as sediments deep in the ocean, soil deep below the Earth's surface and even inside of glacial ice. One genus, Rhizobiales creates nitrogen through nitrogen fixation. This means it can take atmospheric nitrogen (N2 gas) and make it into ammonia (NH3).

    Alphaproteobacteria contains rickettsias that causes Rocky Mountain Spotted Fever.  Rickettsias are obligate intracellular pathogens, which means that they must invade a host cell in order to complete part of their life cycle. 

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   FYI -  Rickettsia spp. includes a lot of different bacteria that causes serious illness in humans, such as R. rickettsii which causes Rocky Mountain spotted fever.  Rocky Mountain Spotted Fever is a bacteria that can be spread to humans, when bitten by an infected tick. The infection causes inflammation of the delicate membranes that surround the brain (meningoencephalitis) and often leads to death.  

Rocky Mountain Spotted Fever

1. The mitochondria in eukaryotic cells were most likely from the proteobacteria phylum as well. You may want to review the endosymbiotic theory below. 

Beta Proteobacteria

Gonorrhea

These bacteria are important for the nitrogen cycle.  Nitrosomonas is a genus of rod-shaped chemoautotrophic bacteria. This organism oxidizes ammonia into nitrite as a metabolic process.

    Betaproteobacteria are a class of proteobacteria which are eutrophs (or copiotrophs), which means that they require a large amount organic nutrients to survive.
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​    The betaproteobacteria includes the bacteria N. gonorrhoeae that causes gonorrhea, and N. meningitides, that causes bacterial meningitis. Several species of betaproteobacteria cause illnesses of the respratory tract including  pertussis (whooping cough). 

Gamma Proteobacteria

    The gamma proteobacteria class includes the species P. aeruginosa which is a highly motile, aerobic, nonfermenting bacterium. Many of these bacteria are symbiotic to humans, however, some of these can cause a number of infections in humans. P. aureginosa causes infections of the urinary tract, the respiratory tract and open wounds. These bacteria are resilient and resistant to many antibiotics.

    Gamma proteobacteria class includes the family enterobacteriaceae. These are intestinal bacteria that are fermenting, facultative anaerobes. This family is made up of two categories; 1) coliforms and 2) noncoliforms.

• Coliforms are able to ferment lactose completely into acid and gas. An example of a coliform bacteria is E. Coli. and Vibrio cholerae which is a Gram-negative, comma-shaped coliform bacterium that causes cholera. 
• Noncoliforms either 1) cannot ferment lactose, or 2) ferment it incompletely into either acid or gas, but not both. An example of a noncoliform bacteria is Salmonella spp. (food poisoning and typhoid),  and Yersinia pestis (bubonic plague).

Delta Proteobacteria

    The last class in the proteobacteria phylum is the delta proteobacteria. These bacteria are gram-negative sulfate-reducing bacteria (SRBs). However, the SRB Desulfovibrio orale does cause gum disease. 

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Epsilon Proteobacteria

     Epsilon proteobacteria includes many bacteria that find their home in the digestive system of the human body. One of these, H. Pylori is the bacteria responsible for stomach ulcers. 

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Cyanobacteria

​   The Earth formed over 4.5 billion years ago. The early atmosphere was oxygen-free, and was composed of carbon dioxide, nitrogen gas and water vapor. ​At some point in our early Earth's history, the Earth cooled enough for the water vapor to liquefy covering the Earth with precious life-giving water.

Fossil of 3.5 Billion Year Old Cyanobacteria

The earliest fossil evidence that we have of life on our planet is over 3.5 billion years old. The earliest known life-form was a single-celled organism called cyanobacteria. 

​   Cyanobacteria are among the first organisms to exist on our planet. They are unicellular, yet can exist as a filament, or as part of a colony. After cyanobacteria were first discovered, they were mis-named, "blue-green algae" due to their coloring. However, these organisms are not algae. Algae are eukaryotic organisms, whereas cyanobacteria are prokaryotic organisms. Also to note, cyanobacteria are not always colored "blue-green. They may also be black, yellow, green, or red.

    Cyanobacteria are able to undergo both photosynthesis and chemosynthesis to meet their energy needs. Both of these processes use up carbon dioxide from the atmosphere and release oxygen as a waste product.  Over the course of hundreds of millions of years, trillions of cyanobacteria transformed the the early Earth's atmosphere to contain the oxygen we breathe. The addition of oxygen to the atmosphere paved the way for new life forms to evolve that depended on oxygen, like humans do. ​​

PHOTOSYNTHESIS

Photosynthesis uses up carbon dioxide and releases oxygen

Chemosynthesis uses up carbon dioxide and releases oxygen

Cyanobacteria can
Photosynthesize and Chemosynthesize

• Photosynthesis and chemosynthesis are both processes by which organisms produce food.
  
• Photosynthesis is converts sunlight's energy (photons) into biologically useful energy (ATP).
  
• Chemosynthesis converts chemical energy from inorganic substances into biologically useful energy (ATP).

  The organisms belonging to the phylum Cyanobacteria are able to perform photosynthesis.  The cyanobacteria perform photosynthesis by using the photosensitive pigment chlorophyll. The chloroplasts of eukaryotic cells were probably cyanobacteria.

Chlamydiae

  Members of the Chlamydiae phylum are so small that they cannot be seen with the light microscope. Chlamydiae is a bacterial phylum and class whose members are non-motile obligate intracellular pathogens. They are ovoid in shape and stain Gram-negative. 

Spirochetes

   Species in the phylum Spirochaetes are thin, spiral‐shaped highly motile bacteria that are Gram‐negative, due to the fact that they have an inner and an outer membrane separated by a peptidoglycan‐containing periplasmic space.

 The spirochaetes include the disease‐causing species Treponema pallidum that causes syphilis, Leptospira sp. that causes leptospirosis and Borrelia burgdorferi that causes Lyme disease. ​

     The bacterial phylum Spirochaetes has many spiral gram-negative bacterial species. Some are found in soil ans others in the G.I. tract of animals.  ​

GRAM-POSITIVE AND GRAM NEGATIVE BACTERIA

Another way that bacteria are categorized, is
​by the way they stain (or if they stain) with the Gram-stain.

​

• Bacteria are divided into two groups; Gram-Positive and Gram Negative, depending on how they react with the Gram stain.​​

  Gram-positive bacteria have a cell wall composed of THICK peptidoglycan which retains a purple/blue hue when stained with gram stain.

Gram-positive Bacteria are found on the skin and mucous membranes.

• Pneumonia
  
• Sinusitis
  
• Cellulitis
  
• Osteomyelitis
  
• Wound infection
  
• Line infection

 Gram-negative bacteria have a cell wall composed of THIN peptidoglycan which does not retain the purple dye (crysal violet) in the Gram stain. They do turn red/pink when exposed to safranin stain in Gram staining.

Gram-negative Bacteria are found in the Gastro-intestinal tract and the urogentital tract. These bacteria are responsible for the following illnesses:

• UTI
  
• Peritonitis
  
• Biliary infection
  
• Pancreatitis
  
• PID - Pelvic Inflammatory Disease

 

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ATYPICAL BACTERIA

  Not all bacteria are able to be stained using Gram staining. Those bacteria are classified as "atypical". For example, bacteria that do not have a cell wall, are not able to be stained by the Gram stain.

​Atypical bacterial infections usually occur in the respiratory and urogenital tract. Atypical bacteria cause
​

• Pneumonia
• Urethritis
• PID - Pelvic Inflammatory Disease

Lungs Infected with Pneumonia

Bacterial and Archaeal Cells 
Shapes and Arrangements

Rod-Shaped Cells are Called Bacilli

 A  cell with a rod shape is called a bacillus (or bacilli, pl.). When only one rod-shaped cell exists, it is called a bacillus.​  ​

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Most rods occur by themselves or in pairs called diplobacillus. 

They can also be arranged into a long chain called strepto-bacillus (strepto = “chains”) ​

Round-Shaped Bacteria are Called Cocci

     A round-shaped bacterial cell is called a coccus (or cocci, pl.). This word comes from the word meaning “berry”, which makes since due to its shape. They do not have to be a perfect sphere. They can be oval or lopsided. They measure only 0.5 µm to 1.0 µm in diameter. 

   Cocci can also form a cube-like tetrad, composed of 4 cocci. 

   Many cocci arrange themselves in pairs called a diplococcus. The organism that causes gonorrhea, Neisseria gonorrhoeae, and one type of bacterial meningitis (N. meningitidis) are diplococci.

    When cocci form an 8-cocci cube, this is called a sarcina (sarcina = “bundle”). ​​​

    Cocci that form a chain are called streptococcus, like the bacteria that causes strep throat (Streptococcus pyogenes) and the bacteria responsible for tooth decay (S. mutans). Other streptococcal species are harmless enough to be used for producing dairy products such as yogurt (S. lactis).

The vibrio is a curved rod that resembles a comma. The cholera-causing pathogen Vibrio cholerae is typical.

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   Sometimes they just cluster together in a random (non-cubed) formation, which is called staphylococcus (staphylo = “cluster”). A well-known example, Staphylococcus aureus, is often a cause of food poisoning, toxic shock syndrome, and several skin infections. The latter are known in the modern vernacular as “staph” infections. 

Spirillum (pl., spirilla) has a helical shape with a thick, rigid cell wall and flagella for motility. 

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COCCUS 

 The spiral-shaped cells called spirochetes have thin, flexible cell walls. Spirochetes contain a diderm (double-membrane).  Spirochetes stain gram-negative and are chemoheterotrophic   Spirochaetes are distinguished from other bacterial phyla by the location of their flagella, sometimes called axial filaments, which run between the bacterial inner membrane and outer membrane in periplasmic space. These cause a twisting motion which allows the spirochaete to move about. 

Harmful Bacteria -
​ (Parasitic Bacteria)

   A parasitic symbiotic relationship is one in which one organism is benefited while the other organism is harmed. There are many species of bacteria that act as parasites to humans and other organisms. These parasitic bacteria are called "pathogenic", because cause diseases. The invading bacteria have mechanisms that evade the host's immune system. They then multiply and occupy space and consume nutrients at great cost to the host. In addition, these parasitic bacteria produce toxic substances (endotoxins and exotoxins) that produce symptoms of the disease in the host. 

    Many species (strains) of bacteria are known to cause illnesses and disease. Some of these include cholera, syphilis, leprosy, anthrax and the bubonic plague (otherwise known as "the black death"). Developed countries combat these bacterial infections using antibiotics. Developing countries, however, still encounter millions of deaths per year due to bacterial infections such as tuberculosis. 

Helpful Bacteria (Mutualistic Bacterial)

 Bacteria are an integral (and even necessary) part of the human body. It is estimated that the human body itself consists of 10 times more bacteria cells than human cells! That comes to a whopping 100-trillion bacterial cells. The portion of our bodies that consist of these bacterial cells has been coined "the human microbiome".  

Humans and other animals have a mutualistic symbiotic relationship with many species of bacteria that make their home in the our mouth, nose, throat, and intestines. 

    A mutualistic relationship is beneficial to both organisms involved. In this situation, the bacteria benefit from using the host as a habitat that provides the nutrients bacteria need to survive. The human (or animal) benefits from this relationship, because the 'good' bacteria helps prevent harmful bacterial or fungal infections. 

​    In fact, co-existing with certain bacteria is SO necessary for good heath, that a billion-dollar industry has formed providing these bacteria to consumers in the form of pills labelled "probiotics"! 

​Illustration Courtesy of alivebynature.com/lactobacillus-gasseri-the-weight-loss-probiotic 

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​   Good bacteria help to defend against the invasion of bad bacteria by populating vulnerable areas (such as the mucosa of body cavities) and sequestering the available resources. 

    Healthy bacteria reside in the vagina that function to regulate the growth of yeast and, thereby, help protect against yeast infections.

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   Bacteria reside on our skin, in our mouth, in our lungs, in our nose, on our eyelashes and in in our eyes!  

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    Healthy bacteria in our gut allow for the proper digestion of food, for the movement of waste through the bowels and for proper elimination of waste.
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Bacteria work to ​convert the nitrogen into nitrates which allows plants to grow!  The plants then provide food for the consumers of that             ecosystem. 

   In addition to living in the ground and the water, prokaryotic microorganisms are abundant in the air, even high in the atmosphere. 

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  Bacteria that love salt are called "halobacteria". The halobacteria that live in the Dead Sea function to recycle the remains of dead brine shrimp into biologically useful molecules. 

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     Bacteria can even change their food source!  In other words, they are "metabolically flexible."  For example, some types of bacteria can metabolize carbohydrates, sulfates, hydrogen ion etc.

   Salmonella is a genus of bacteria belonging to the noncliform group of Enterobactericeae, Many serotypes of Salmonellacan cause food poisoning (salmonellosis), ehich causes fever, nausea, vomiting, and diarrhea.