There are two major categories of bacteria on the basis of cell wall structure: Gram positive bacteria have thick walls with a dense homogeneous layer of peptidoglycan (a complex polysaccharide polymer composed of two types of sugar subunit, - we look at this later) Gram negative bacteria have thin cell walls with much less peptidoglycan and an EXTRA outer lipid membrane (not a cell membrane) that contains a toxic compound called lipopolysaccharide (LPS).
A Gram positive bacterium with a thick exposed layer of peptidoglycan. Gram positive walls also contain a unique substance called teichoic acid
A Gram negative bacterium with an extra outer lipid membrane covering a thin layer of peptidoglycan. The outer membrane contains lipopolysaccharide (LPS) also known as endotoxin
The term “Gram stain” refers to a dye staining procedure involving sequential treatment of bacteria on a slide with crystal violet dye, iodine, alcohol and safranin dye. We will look at this in detail in a later tutorial. The completed Gram stain has one of two results, the dyed cell is red when viewed under the microscope and is termed as Gram negative, or it is purple when viewed under the microscope and is termed as Gram positive. The particular Gram result is correlated with a particular cell wall structure, and it is the iodine treatment step that distinguishes between Gram positive and Gram negative, but the alcohol is the point where Gram positive and Gram negative are differentiated. The nature of the cell wall, Gram negative or Gram positive, has a high relevance to the general type of human disease the bacterium might cause and what kind of antibiotic might be used to treat it, and thus the Gram stain is the FIRST bit of knowledge one needs when analyzing a bacterial infection. For instance, many skin and wound infections are caused by gram positive bacteria, whereas many gut dwelling bacteria are gram negative. There are also a few bacteria which do not stain well by the Gram stain, such as the acid-fast bacteria, they have a lot of waxy lipid in their cell walls and capsules that interferes with the Gram stain. The tuberculosis and leprosy causing genus Mycobacterium which we will look at later, is an example of an acid fast bacterium which is initially identified in sputum or other body fluids by means of an acid-fast dye staining procedure. As mentioned, very few bacterial species lack a cell wall, Mycoplasma species are the best example, these commonly infect meat and are nuisance contaminants of cell cultures in research laboratories, Mycoplasma cannot be identified with a gram stain, they will accept the dyes involved, but the result is meaningless in the absence of a cell wall. All cells, prokaryotic and eukaryotic, contain ribosomes (non-membrane bound organelles) which synthesize proteins, but those of the prokaryotes are smaller, this is an important consideration in treating bacterial infections, some antibiotics such as tetracycline attack and inactivate the smaller bacterial ribosome but not our larger eukaryotic ribosome. Ribosomes are complex macro (very large) molecules composed of a smaller and a larger subunit, they consist of a special nucleic acid (ribosomal nucleic acid) and protein and their function is to synthesize proteins by reading the mRNA copy of sections of DNA (more detail later on this topic). Ribosomes are organelles but they are NOT membrane bound. There are two sizes of ribosomes, 80S in eukaryotic cells and 70S in bacteria and mitochondria (don’t worry about what is meant by 80S and 70S, just understand 80S is heavier than 70S). It is no accident that mitochondria have bacterial sized ribosomes - mitochondria evolved from bacteria. Bacteria may have flagella, which are whip like structures that extend through the cell wall from the cell membrane, they allow movement, and some bacteria have much smaller flexible rod like extensions which are called pili (not found in eukaryotic cells) and function in reproduction and adherence to surfaces. When bacteria can move, either by means of flagella or a process called gliding which does not involve flagella, they are referred to as being motile. Many bacteria have a thick layer called a capsule, which can help the bacteria to evade immune detection and make them more virulent (able to cause disease more aggressively).