4.1 Gram-positive and Gram-negative bacteriaBacteria are divided into two groups based on how the cell wall appears when they are stained using Gram straining. This procedure allows the composition of the wall to be visualised. Show In Gram-positive bacteria, the cell wall has a thick peptidoglycan layer which is relatively porous, allowing substances to pass through it quite easily. In Gram-negative bacteria, this peptidoglycan layer is greatly reduced and is further protected by a second, outer membrane (Figure 10). Figure 10 Arrangement of the cell wall in (a) Gram-positive and (b) Gram-negative bacteria. Show description|Hide description This diagram shows the differences in cell wall structure between Gram-positive and Gram-negative bacteria. In the Gram-positive bacteria in (a) the peptidoglycan is a thick external layer shown in brown, while in the Gram-negative bacteria in (b) the peptidoglycan layer is much thinner and is surrounded by an outer membrane of lipopolysaccharide and protein (as a green wavy line). The inner membrane in (a) and (b) (shown as a double green line) is separated from the peptidoglycan layer by the periplasmic space. Figure 10 Arrangement of the cell wall in (a) Gram-positive and (b) Gram-negative bacteria. This second, outer membrane of Gram-negative bacteria is an effective barrier, regulating the passage of large molecules such as antibiotics into the cell. In contrast, the thick, porous peptidoglycan layer in the cell wall of Gram-positive bacteria gives greater access to antibiotics, allowing them to more easily penetrate the cell and/or interact with the peptidoglycan itself. You will learn more about the strategies antibiotics use to cross the cell wall in Week 3. Staining and IdentificationMicroscopic view of dental plaque, showing Gram-positive (purple) and negative (red) bacteria In a Gram stain test, bacteria are washed with a decolorizing solution after being dyed with crystal violet. On adding a counterstain such as safranin or fuchsine after washing, Gram-negative bacteria are stained red or pink while Gram-positive bacteria retain their crystal violet dye. This is due to the difference in the structure of their bacterial cell wall. Gram-positive bacteria do not have an outer cell membrane found in Gram-negative bacteria. The cell wall of Gram-positive bacteria is high in peptidoglycan which is responsible for retaining the crystal violet dye. Gram-positive and negative bacteria are chiefly differentiated by their cell wall structure The following videos demonstrate the staining of Gram-positive and negative bacteria respectively. Pathogenesis in humansBoth gram-positive and gram-negative bacteria can be pathogenic (see list of pathogenic bacteria). Six gram-positive genera of bacteria are known to cause disease in humans: Streptococcus, Staphylococcus, Corynebacterium, Listeria, Bacillus and Clostridium. Another 3 cause diseases in plants: Rathybacter, Leifsonia, and Clavibacter. Many gram-negative bacteria are also pathogenic e.g., Pseudomonas aeruginosa, Neisseria gonorrhoeae, Chlamydia trachomatis, and Yersinia pestis. Gram-negative bacteria are also more resistant to antibiotics because their outer membrane comprises a complex lipopolysaccharide (LPS) whose lipid portion acts as an endotoxin. They also develop resistance sooner: A lot of Gram-negative bacteria, they come out of the box, if you will, resistant to a number of important antibiotics that we might use to treat them. We’re talking about agents with names like Acinetobacter, Pseudomonas, E. coli. These are bacteria that have historically done a very good job of very quickly developing resistance to antibiotics. They have a lot of tricks up their sleeves for developing resistance to antibiotics, so they’re a group of agents that can quickly become resistant, can pose major challenges to resistance. And what we’ve seen over the past decade is these Gram-negative agents becoming very rapidly more and more resistant to all of the agents that we have available to treat them. Greater resistance of gram-negative bacteria also applies to a newly discovered class of antibiotics that was announced in early 2015 after a decades-long drought in new antibiotics. These drugs are not likely to work on gram-negative bacteria. Structure of a gram-positive bacterial cell. Gram positive CocciBacteria are classified based on their cell shape into bacilli (rod shaped) and cocci (sphere shaped).Typical Gram-positive cocci stains include (pictures):
Gram-positive bacilli tend to be thick, thin or branching. Commercial uses of non-pathogenic Gram-positive bacteriaMany streptococcal species are nonpathogenic, and form part of the commensal human microbiome of the mouth, skin, intestine, and upper respiratory tract. They are also a necessary ingredient in producing Emmentaler (Swiss) cheese. Non-pathogenic species of corynebacterium are used in industrial production of amino acids, nucleotides, bioconversion of steroids, degradation of hydrocarbons, cheese ageing, production of enzymes etc. Many Bacillus species are able to secrete large quantities of enzymes.
The anaerobic bacterium C. ljungdahlii can produce ethanol from single-carbon sources including synthesis gas, a mixture of carbon monoxide and hydrogen that can be generated from the partial combustion of either fossil fuels or biomass. Gram-indeterminate and Gram-variable BacteriaNot all bacteria can be reliably classified through Gram staining. For example, acid-fast bacteria or Gram-variable do not respond to Gram staining. References
Which bacteria is most antibiotic resistant?MRSA is one of the most common antibiotic-resistant bacteria.
Why is gramGram-positive bacteria, those species with peptidoglycan outer layers, are easier to kill - their thick peptidoglycan layer absorbs antibiotics and cleaning products easily. In contrast, their many-membraned cousins resist this intrusion with their multi-layered structure.
Why most of the GramExample: Gram-negative bacteria have an outer layer (membrane) that protects them from their environment. These bacteria can use this membrane to selectively keep antibiotic drugs from entering.
Is GramAntibiotic resistance has become a major problem in the treatment of gram-positive bacterial infections.
|