The proteobacteria are a major phylum of gram-negative bacteria, including Escherichia coli (E. coli), Salmonella, Shigella, and other Enterobacteriaceae, Pseudomonas, Moraxella, Helicobacter, Stenotrophomonas, Bdellovibrio, acetic acid bacteria, Legionella etc.
Cocci refer to bacteria which are spherical, whereas Rods are rod shaped. Gram positive bacteria have a thick layer of peptidoglycan which holds the Crystal Violet stain used in Gram Staining and so appear purple under the microscope. Gram negative bacteria have a thin layer of peptidoglycan held between two membranes.
Many Gram-negative germs live in the human digestive tract (colon). Some Gram-negative germs can be found on the skin of healthy people. Not every one gets an illness from Gram-negative germs. But if these germs invade parts of the body where they are not commonly found, they can cause serious illness.
Tetracyclines are broad-spectrum antibiotics, active against both Gram-positive and Gram-negative bacteria. Their use is decreasing to increasing instances of bacterial resistance; however, they still find use in treatment of acne, urinary tract, and respiratory tract infections, as well as chlamydia infections.
Gram-negative: Gram-negative bacteria lose the crystal violet stain (and take the color of the red counterstain) in Gram's method of staining. This is characteristic of bacteria that have a cell wall composed of a thin layer of a particular substance (called peptidoglycan).
Escherichia coli (commonly abbreviated E. coli) is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms (endotherms). Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans.
Example species. The proteobacteria are a major phylum of gram-negative bacteria, including Escherichia coli (E. coli), Salmonella, Shigella, and other Enterobacteriaceae, Pseudomonas, Moraxella, Helicobacter, Stenotrophomonas, Bdellovibrio, acetic acid bacteria, Legionella etc.
E coli is a gram-negative bacillus that grows well on commonly used media. It is lactose-fermenting and beta-hemolytic on blood agar. Most E coli strains are nonpigmented. The image below shows Escherichia coli on Gram staining.
Gram-positive bacteria retain the crystal violet dye, and thus are stained violet, while the Gram-negative bacteria do not; after washing, a counterstain is added (commonly safranin or fuchsine) that will stain these Gram-negative bacteria a pink color.
These antibiotics include cephalosporins (ceftriaxone-cefotaxime, ceftazidime, and others), fluoroquinolones (ciprofloxacin, levofloxacin), aminoglycosides (gentamicin, amikacin), imipenem, broad-spectrum penicillins with or without β-lactamase inhibitors (amoxicillin-clavulanic acid, piperacillin-tazobactam), and
Characteristics of Gram-negative Bacteria. Gram-negative bacteria have a characteristic cell envelope structure very different from Gram-positive bacteria. Gram-negative bacteria have a cytoplasmic membrane, a thin peptidoglycan layer, and an outer membrane containing lipopolysaccharide.
Gram-positive bacteria, those species with peptidoglycan outer layers, are easier to kill - their thick peptidoglycan layer absorbs antibiotics and cleaning products easily. As a result, Gram-negative bacteria are not destroyed by certain detergents which easily kill Gram-positive bacteria.
MRSA, which most everyone knows about now, is gram-positive. We know about MRSA, but there has been an increase in infections caused by gram-negative bacteria, and they are resistant to many, or sometimes all, drugs.
Although all bacteria have an inner cell membrane, gram-negative bacteria have a unique outer membrane. This outer membrane excludes certain drugs and antibiotics from penetrating the cell, partially accounting for why gram-negative bacteria are generally more resistant to antibiotics than are gram-positive bacteria.
Gram-negative bacilli such as Enterobacteriaceae and Pseudomonas aeruginosa are the leading causes of nosocomial bloodstream infections. This marked increase in the incidence of infections due to antibiotic-resistant gram-negative bacilli in recent years is of great concern.
As a rule of thumb (which has exceptions), Gram-negative bacteria are more dangerous as disease organisms, because their outer membrane is often hidden by a capsule or slime layer which hides the antigens of the cell and so acts as "camouflage" - the human body recognises a foreign body by its antigens; if they are
In the classical sense, six Gram-positive genera are typically pathogenic in humans. Two of these, Streptococcus and Staphylococcus, are cocci (sphere-shaped). The remaining organisms are bacilli (rod-shaped) and can be subdivided based on their ability to form spores.
Gram-negative bacteria cause infections including pneumonia, bloodstream infections, wound or surgical site infections, and meningitis in healthcare settings. Gram-negative infections include those caused by Klebsiella, Acinetobacter, Pseudomonas aeruginosa, and E. coli., as well as many other less common bacteria.
Coccobacilli, then, are very short rods which may be mistaken for cocci. Haemophilus influenzae, Gardnerella vaginalis, and Chlamydia trachomatis are coccobacilli. Aggregatibacter actinomycetemcomitans is a Gram-negative coccobacillus prevalent in subgingival plaques.
P. aeruginosa is a Gram-negative, aerobic (and at times facultatively anaerobic), bacillus with unipolar motility. It has been identified as an opportunistic pathogen of both humans and plants. P. aeruginosa is the type species of the genus Pseudomonas.
Staphylococci are gram-positive aerobic organisms. Staphylococcus aureus is the most pathogenic; it typically causes skin infections and sometimes pneumonia, endocarditis, and osteomyelitis. S. lugdunensis, a coagulase-negative species, can cause invasive disease with virulence similar to that of S. aureus.