In general, viruses are more difficult to kill than bacteria, but the disinfection step in water treatment is adequate to prevent either type of germ from reaching the consumer.
Viruses are the smallest and least complex of the microbes, these entities are more like crystals of protein and nucleic acid rather than living cells, and cannot reproduce except inside host cells. In 1898, Friedrich Loeffler and Paul Frosch found evidence that the cause of foot-and-mouth disease in livestock was an infectious particle smaller than any bacteria. This was the first clue to the nature of viruses, genetic entities that lie somewhere in the grey area between living and non-living states. Viruses depend on the host cells that they infect to reproduce. When found outside of host cells, viruses exist as a protein coat or capsid, sometimes enclosed within a membrane. The capsid encloses either DNA or RNA which codes for the virus elements. While in this form outside the cell, the virus is metabolically inert.
When it comes into contact with a host cell, a virus can insert its genetic material into its host, literally taking over the host's functions. An infected cell produces more viral protein and genetic material instead of its usual products. Some viruses may remain dormant inside host cells for long periods, causing no obvious change in their host cells (a stage known as the lysogenic phase). But when a dormant virus is stimulated, it enters the lytic phase: new viruses are formed, self-assemble, and burst out of the host cell, killing the cell and going on to infect other cells. The diagram below at right shows a virus that attacks bacteria, known as the lambda bacteriophage, which measures roughly 200 nanometers.
Bacteria are bigger than viruses, but smaller than (and somewhat different in structure and function from) the other microbes, these single-celled entities are still quite complex and are generally able to obtain nutrients and energy from their environment and reproduce on their own.
A more or less typical bacterium, shown here, is comparatively much simpler than a typical eukaryotic cell. View the transmission electron micrograph of a typical bacterium, E. coli, below and compare it with the diagram above. Bacteria lack the membrane-bound nuclei of eukaryotes; their DNA forms a tangle known as a nucleoid, but there is no membrane around the nucleoside, and the DNA is not bound to proteins as it is in eukaryotes. Whereas eukaryote DNA is organized into linear pieces, the chromosomes, bacterial DNA forms loops. Bacteria contain plastids, or small loops of DNA, that can be transmitted from one cell to another, either in the course of sex (yes, bacteria have sex) or by viruses. This ability to trade genes with all comers makes bacteria amazingly adaptable; beneficial genes, like those for antibiotic resistance, may be spread very rapidly through bacterial populations. It also makes bacteria favorites of molecular biologists and genetic engineers; new genes can be inserted into bacteria with ease. Bacteria do not contain membrane-bound organelles such as mitochondria or chloroplasts, as eukaryotes do.
However, photosynthetic bacteria, such as cyanobacteria, may be filled with tightly packed folds of their outer membrane. The effect of these membranes is to increase the potential surface area on which photosynthesis can take place. The cell membrane is surrounded by a cell wall in all bacteria except one group, the Mollicutes, which includes pathogens such as the mycoplasmas. The composition of the cell wall varies among species and is an important character for identifying and classifying bacteria. In this diagram, the bacterium has a fairly thick cell wall made of peptidoglycan (carbohydrate polymers cross-linked by proteins); such bacteria retain a purple color when stained with a dye known as crystal violet, and are known as Gram-positive (after the Danish bacteriologist who developed this staining procedure). Other bacteria have double cell walls, with a thin inner wall of peptidoglycan and an outer wall of carbohydrates, proteins, and lipids. Such bacteria do not stain purple with crystal violet and are known as Gram-negative.
Protozoan and their cysts are generally bigger than fungal cells and very much like human cells in structure and function, these single-celled entities are quite able to obtain nutrients and energy from their environment and reproduce on their own, sometimes via sexual means. Protozoan cysts are the hardest to kill, with Cryptosporidium being harder to kill than Giardia. Cryptosporidium is so hard to kill that water suppliers who use surface water as their source depend on filtration to remove this germ, instead of trying to kill it.