Viruses vs. Bacteria: Biology, Structure, and Function

Introduction to Viruses

A virus is a small particle that infects cells in biological organisms. Viruses are obligate intracellular parasites; they can reproduce only by invading and controlling other cells, as they lack the cellular machinery for self-reproduction. The term virus usually refers to those particles that infect eukaryotes (multi-celled organisms and many single-celled organisms), while the term bacteriophage or phage is used to describe those infecting prokaryotes (bacteria and bacteria-like organisms).

Typically, these particles carry a small amount of nucleic acid (either DNA or RNA) surrounded by a protective coat consisting of proteins, lipids, and glycoproteins. Importantly, viral genomes code not only for the proteins needed to package genetic material, but also for proteins needed by the virus during its lysogenic and lytic cycles — the two primary reproductive cycles. A virus reproduces by causing a host cell to create copies of itself. When found outside of a host cell, viruses consist of genomic nucleic acid (either DNA or RNA) surrounded by a protein coat, or capsid, with or without a glycoprotein envelope.

There are three main types of viruses: the bacterial virus (otherwise called a bacteriophage), the animal virus, and the retrovirus. The complete virus particle is referred to as a virion. A virion is little more than a gene transporter; components of the envelope and capsid provide the mechanism for injecting the viral genome into a host cell.^[1] The number of proteins required to form a spherical virus capsid is denoted by the "T-number," whereby 60T proteins are necessary. In the case of the hepatitis B virus, the T-number is 4, meaning 240 proteins assemble to form the capsid. As with helical viruses, the spherical virus capsid may be enclosed in a lipid envelope, although spherical viruses are frequently non-enveloped, with the capsid proteins themselves directly involved in attachment and entry into the host cell.

Bacteriophages: Viruses That Infect Bacteria

A phage (also called a bacteriophage) is a small virus that infects only bacteria. Like viruses that infect eukaryotes, phages consist of an outer protein hull enclosing genetic material — which consists of double-stranded DNA in 95 percent of known phages — ranging from 5 to 650 kbp (kilobase pairs) in size and 24 to 200 nm in length. The vast majority of phages (95 percent) have a tail that allows them to inject their genetic material into the host. Phages were discovered independently by Frederick Twort in 1915 and by Félix d'Hérelle in 1917.

Phages infect only specific bacteria. Some phages are virulent, meaning that upon infecting a cell they immediately begin reproducing, and within a short time they lyse (destroy) the cell, releasing new phages. Some phages — so-called temperate phages — can instead enter a relatively harmless state, either integrating their genetic material into the chromosomal DNA of the host bacterium (much like endogenous retroviruses in animals) or establishing themselves as plasmids. These endogenous phages, referred to as prophages, are then copied with every cell division alongside the host cell's DNA. They do not kill the cell, but monitor the status of their host via certain proteins they encode. When the host cell shows signs of stress — suggesting it may be close to death — the endogenous phages become active again and initiate their reproductive cycle, resulting in lysis of the host cell.

Sometimes, prophages even provide a benefit to the host bacterium while dormant, by adding new functions to the bacterial genome — a phenomenon called lysogenic conversion. A well-known example is the harmless Vibrio bacteria strain, which is converted into Vibrio cholerae by a phage, thereby causing cholera. Phages also play an important role in molecular biology as cloning vectors for inserting DNA into bacteria. Phage therapy has been used since the 1940s in the former Soviet Union as an alternative to antibiotics for treating bacterial infections. An extensive library of research into specific phages and their therapeutic uses is maintained at the Tbilisi Institute in Georgia. The development of bacterial strains resistant to multiple drugs has led Western medical researchers to re-evaluate phages as alternatives to antibiotics.

Animal Viruses and Retroviruses

Animal viruses replicate similarly to bacteriophages, but with some modifications. If the virus has an envelope, glycoprotein spikes allow it to adhere to plasma membrane receptors. The viral genome, covered by the capsid, then penetrates the host cell. Once inside, the virus is uncoated as the envelope and capsid are removed. Free of its covering, the viral genome (DNA or RNA) proceeds with biosynthesis. Newly assembled viral particles are released by budding. Components of viral envelopes — lipids, proteins, and carbohydrates — are obtained from the plasma or nuclear membrane as the viruses leave the cell. Budding does not necessarily kill the host cell.

A retrovirus is a virus whose genome consists of two positive-sense RNA molecules, which may or may not be identical. It relies on reverse transcriptase to perform the reverse transcription of its genome from RNA into DNA, which can then be integrated into the host's genome by an integrase enzyme. The virus itself serves as a storage form for its nucleic acid genome as well as a means of delivering its genome into target cells. Once inside the host cell, the RNA strands undergo reverse transcription in the cytosol. Once integrated into the host's genome, the retroviral DNA is referred to as a provirus.

While transcription was classically understood to occur only from DNA to RNA, reverse transcriptase transcribes RNA into DNA. The term "retro" in retrovirus refers to this reversal of the central dogma of molecular biology. Reverse transcriptase activity outside of retroviruses has been found in almost all eukaryotes, enabling the generation and insertion of new copies of retrotransposons into the host genome.

Because reverse transcription lacks the usual "proofreading" associated with DNA transcription, this class of virus mutates very frequently. This enables the virus to rapidly develop resistance to antiviral pharmaceuticals, and is one of the main reasons why an effective vaccine for HIV has not yet been developed. Studies of retroviruses led to the first demonstrated synthesis of DNA from RNA templates — a fundamental mode of transferring genetic material that occurs in both eukaryotes and prokaryotes. Certain researchers speculate that the processes followed by retroviruses (RNA → DNA → RNA → Protein) may be key to understanding the evolution of DNA itself — that in the "primordial soup," retroviruses evolved to create DNA from RNA templates, which was subsequently adopted by cellular organisms due to the increased chemical stability of DNA.

Introduction to Bacteria

As one source puts it: "Viruses can be considered to be on the threshold between living and non-living. In a sense, they can cause themselves to be reproduced, so they could be considered as being 'alive.' But they can only do this by using the machinery of a host cell. Without a host, they exist only as inert, almost crystal-like, 'non-living' particles in the environment."^[2] Viruses have genes and display inheritance, but are entirely reliant on host cells to produce new generations. Many viruses share similarities with complex molecules: like DNA, viruses undergo molecular replication and can often be crystallized. Because viruses depend on host cells for replication, they are generally not classified as "living."

Whether or not they are alive, viruses are obligate parasites and have no form capable of reproducing independently of a host. Like most parasites, they have a specific host range — sometimes limited to one species (or even specific cell types within one species) and sometimes broader. Concerning the question of viral life, if the requirement for autonomous self-reproduction is set aside, a strong argument can be made that viruses are indeed alive. Some small viruses are more functionally efficient than most cellular life forms, as their ratio of functions to working parts is remarkably high. If viruses are considered alive, the prospect of creating artificial life becomes more plausible — or at least the threshold for calling something "artificially alive" is lowered.

Bacteria are a major group of living organisms. Most are microscopic and unicellular, with a relatively simple cell structure that lacks a cell nucleus, cytoskeleton, and organelles such as mitochondria and chloroplasts. The term "bacteria" has been variously applied to all prokaryotes or to a major subset of them, depending on prevailing ideas about their evolutionary relationships.

Bacteria are the most abundant of all organisms. They are ubiquitous in soil, water, and as symbionts of other organisms. In evolutionary terms, bacteria are thought to be among the oldest organisms on Earth, appearing approximately 3.7 billion years ago. Many pathogens are bacteria. Most are minute — usually only 0.5–5.0 μm in size — though one type, Thiomargarita namibiensis, reaches 0.5 mm in diameter and has a volume up to a million times that of the typical bacterium. Bacteria generally have cell walls, like plant and fungal cells, but with a very different composition (peptidoglycans). Many bacteria move using flagella, which differ structurally from the flagella of other groups.

Based on their response to oxygen, most bacteria can be placed into one of three groups. Some bacteria can grow only in the presence of oxygen and are called aerobes; others can grow only in the absence of oxygen and are called anaerobes; and some can grow in either condition and are called facultative anaerobes. Bacteria that do not utilize oxygen for respiration but can still grow in its presence are called aerotolerant. Bacteria also thrive in environments considered extreme for most life. These organisms are called extremophiles. Some bacteria inhabit hot springs (thermophiles), others inhabit highly saline lakes (halophiles), some inhabit acidic or alkaline environments (acidophiles and alkaliphiles, respectively), and others inhabit alpine glaciers (psychrophiles).