Major Bacteria Groups

1. GRAM POSITIVE EUBACTERIA

• These bacteria have a cell wall profile consistent with gram positive bacteria.
• These cells may be encapsulated and can exhibit flagellar-mediated motility. 
• Cells may be spherical, rods, or filaments; the rods and filaments. 
• Reproduction is generally by binary fission, although some bacteria in this category produce spores (may be sporogenous or asporogenous).
• They may also include aerobic, anaerobic, and facultatively anaerobic

2. GRAM NEGATIVE EUBACTERIA

• Bacteria in this group have a complex cell envelope consisting of:`
  •  an outer membrane, 
  • a periplasmic space containing a thin peptidoglycan layer, and 
  • a cytoplasmic membrane.
• The cell shape may be spherical, oval, straight or curved rods, helical, or filamentous
• Reproduction is by binary fission, but some groups reproduce by budding.
• They include aerobic, anaerobic, facultatively anaerobic, and microaerophilic species

3. EUBACTERIA LACKING CELL WALLS

• These are microorganisms that lack cell walls (commonly called mycoplasmas).
• They resemble the L-forms that can be generated from many species of bacteria.
• However, mycoplasmas never revert to the walled state, and there are no antigenic relationships between mycoplasmas and eubacterial L-forms.
• Mycoplasmas are highly pleomorphic organisms and range in size from vesicle-like forms to very small (0.2 μm), filterable forms (meaning that they are too small to be captured on filters that routinely trap most bacteria.
• Reproduction may be by budding, fragmentation, or binary fission, singly or in combination. 
• Most species require a complex medium for growth and tend to form characteristic “fried egg” colonies on a solid medium

1. GRAM POSITIVE EUBACTERIA

A. Bacillus

• They are Gram Positive Eubacteria
• They include large aerobic, gram-positive rods.
• Spores are located in the center of the non motile bacilli.
• They are prevalent in soil, water, and air and on vegetation.
• Of the many species of Bacillus and related genera, most do not cause disease
• Members include: Bacillus cereus, Bacillus thuringiensis and Bacillus subtilis.
• Most members are saprophytic.
• Both B cereus and B thuringiensis may occasionally produce disease in immunocompromised humans (eg, meningitis, endocarditis, endophthalmitis, conjunctivitis, or acute gastroenteritis). 
• B anthracis, which causes anthrax, is the principal pathogen of the genus.

Bacillus anthracis

• Colonies of B anthracis are round and have a “cut glass” appearance in transmitted light.
• Hemolysis is uncommon with B anthracis
• Anthrax is primarily a disease of herbivores—goats, sheep, cattle, horses.
• In animals, the portal of entry is the mouth and the gastrointestinal tract via spores from contaminated soil.
• In humans, the infection is usually acquired by the entry of spores through injured skin (cutaneous anthrax, 95%) or rarely the mucous membranes (gastrointestinal anthrax) or by inhalation of spores into the lung (inhalation anthrax, 5%).
• B. anthracis isolates that do not produce a capsule are not virulent and do not induce anthrax in test animals.
• Specimens to be examined include:blood, pleural fluid, and cerebrospinal fluid; stool or other intestinal contents
• Definitive identification requires:
  •  lysis by a specific anthrax γ-bacteriophage, 
  • detection of the capsule by fluorescent antibody, or 
  • identification of toxin genes by polymerase chain reaction (PCR

Bacillus cereus

• In semi-solid medium, it exhibits motility by “swarming.”
• It is β-hemolysis when culture in blood agar.
• It has been indicated in causing food poisoning.
• A concentration of 105 bacteria or more per gram of food is considered diagnostic.
• B cereus is an important cause of eye infections, such as severe keratitis, endophthalmitis, and panophthalmitis.
• B cereus has also been associated with localized infections and with systemic infections, including endocarditis, meningitis, osteomyelitis, and pneumonia

B. Clostridium

• They are large anaerobic, gram-positive, motile rods.
• They form spores which can be placed centrally, subterminally, or terminally
• Many decompose proteins or form toxins, and some do both.
• Their natural habitat is the soil or the intestinal tract of animalsand humans, where they live as saprophytes. 
• Among the pathogens are the organisms causing botulism, tetanus, gasgangrene, and pseudomembranous colitis.
• Some clostridia produce large raised colonies (eg, C. perfringens);others produce smaller colonies (eg, C. tetani).

Clostridium botulinum

• Most cases of botulism represent an intoxication resulting from the ingestion of food in which C botulinum has grown and produced toxin.
• These food items include spiced, smoked,vacuum packed, or canned alkaline foods that are eaten withoutcooking. In such foods, spores of C botulinum germinate;that is, under anaerobic conditions, vegetative forms grow and produce toxin.
• Symptoms begin 18–24 hours after ingestion of the toxic food,with:
  • visual disturbances (incoordination of eye muscles,double vision), 
  • inability to swallow, and speech difficulty;
  • signs of bulbar paralysis are progressive, and 
  • death from respiratory paralysis or cardiac arrest.
• Toxin can often be demonstrated in serum, gastric secretions,or stool from the patient, and toxin may be found in leftoverfood

Clostridium tetani

• C tetani, which causes tetanus can be found in the soil and in the feces of horses and other animals.
• C tetani is not an invasive organism. The infection remainsstrictly localized in the area of devitalized tissue (wound,burn, injury, umbilical stump, surgical suture) into whichthe spores have been introduced.
• The toxin degrades synaptobrevin, a protein required for docking of neurotransmittervesicles on the presynaptic membrane.
• In addition, release of the inhibitory glycine and GABA is blocked, and the motor neurons are not inhibited. Hyperreflexia, muscle spasms, and  paralysis result.
• The bacteria can isolated from anaerobic culture of tissues from contaminated wounds

C. Staphylococcus

• They are gram-positive, aerobic cocci, arranged in irregular clusters.
• Staphylococci are nonmotile and do not form spores.
• They are found free living in the environment and colonies, when cultured, appear yellow, red, or orange.
• Colonies on solid media are round, smooth,raised, and glistening.
• They grow readily on many types of media and are active metabolically, fermenting carbohydrates and producing pigments that vary from white to deep yellow.
• The staphylococci produce catalase, which differentiates them from the streptococci.
• Staphylococcus species are beta-hemolytic when grown on a blood agar medium.
• Clinically important species include Staphylococcus aureus, Staphylococcus epidermidis, Staphylococcus lugdunensis, and Staphylococcus saprophyticus.

Staphylococcus aureus

• S. aureus usually forms gray to deep golden yellow colonies.
• S. aureus produces coagulase which binds to prothrombin;enzymatically activating the latter to initiate fibrin polymerization.
• S. aureus has been implicated in abscesses, hair follicle infection (pimple), vulvovaginitis, direct wound contamination, pneumonia, meningitis, sepsis, among others.
• Specimens includes swabs of the surface of the affected areas (pus, blood, vaginal swab, etc).

2. GRAM NEGATIVE EUBACTERIA

A. ENTEROBACTERIACEAE

• They are gram-negative rods, either motile with peritrichous flagella or nonmotile.
• They are facultative anaerobes.
• They are catalase positive and oxidase negative (exceptfor Plesiomonas).
• Capsules are large and regular in Klebsiella species, less so in Enterobacter species, and uncommon in the other species.
• The Salmonellae and Shigellae produce colonies similar to E. coli but do not ferment lactose.

Escherichia coli

• It is part of the intestinal normal flora but causes infection when present in other parts of the body. For example, urinary tract, lung, blood, meninges, biliary tract, etc.
• It has been implicated in urinary tract infections, meningitis, diarrhea, among others.
• Specimens include urine, blood, pus, spinal fluid, sputum, orother material, as indicated by the localization of the diseaseprocess.
• E. coli typically produces positive testresults for indole, lysine decarboxylase, and mannitol fermentationand produces gas from glucose.
• E. coli by its hemolysis on blood agar.
• E coli and most of the other enteric bacteria form circular,convex, smooth colonies with distinct edges.

B. VIBRIO

• They are curved aerobic rods and aremotile, possessing a polar flagellum.
• On prolonged cultivation, Vibrios may become straight rods that resemble the gram-negative entericbacteria.

Vibrio cholerae

• Cholera is not an invasive infection. The organisms donot reach the bloodstream but remain within the intestinaltract.
• It is characterized by sudden onset of nausea and vomiting and profuse diarrhea with abdominal cramps.
• Stools, which resemble “rice water,” contain mucus, epithelial cells, and large numbers of vibrios. There is rapid loss offluid and electrolytes.
• Specimens for culture consist of mucus flecks from stools.
• V. cholerae produces convex, smooth, round colonies thatare opaque and granular in transmitted light.

• V. cholera grows well on thiosulfate-citrate-bile-sucrose (TCBS) agar,a media selective for vibrios, on which it produces yellow colonies (sucrose fermented) that are readily visible against the dark-green background of the agar.
• Characteristically, vibrios grow at a very high pH (8.5–9.5) and are rapidly killed by acid.
• A positive oxidase test result is a key step in the preliminary identification of V. cholerae and other vibrios

C. MYCOPLASMA

• Culture of mycoplasmas that cause disease in humans requiresmedia with serum, ametabolic substrate such as glucose orurea, and growth factors such as yeast extract.
• Growth onsolid media consists principally of protoplasmic masses ofindefinite shape that are easily distorted. Th ese structuresvary greatly in size, ranging from 50 to 300nm in diameter.
• Clinically important members include:
• Mycoplasma pneumoniae causes pneumoniaand has been associated with joint and other infections.
• Mycoplasma hominis sometimes causes postpartum fever

Mycoplasma pneumoniae

• M. pneumoniae infection ranges fromasymptomatic infection to serious pneumonitis, with occasionalneurologic and hematologic (ie, hemolytic anemia).
• Typical symptoms include malaise, fever, headache,sore throat, and cough.
• A sputum Gram stain is of value in not suggestingsome other bacterial pathogen (eg, S. pneumoniae)