We learn about viruses and bacteria

Viruses and bacteria are often the cause of infectious diseases in humans, local or systemic, and whether or not associated with fever. Although they have some common characteristics, such as the ability to transmit from one individual to another, and can sometimes induce similar diseases, they are very different microbes.

Bacteria are organisms consisting of a single cell (unicellular) prokaryotes (ie without cell nucleus), but similar to complete living organisms, since they are equipped with all the structures and enzymes necessary to perform the fundamental metabolic functions, thus being able to live independently and reproduce autonomously, by cell division.

Viruses are, on average, 100 times smaller than bacteria (to the point of being invisible to the light microscope), and have neither a complete cellular structure, nor all the enzymes necessary for the survival and replication of their nucleic acid, which can be made up of DNA or RNA (in this second case they are called “retroviruses” and the best known exponent of the group is HIV).

In addition, viruses do not possess ribosomes, which are the cellular organelles essential to produce all the proteins existing in nature, therefore also those that form the viral capsid, corresponding to the envelope of the virus.

This makes viruses unable to reproduce autonomously and means that, in order to multiply and cause infections, they must enter a “complete” host cell (which can be an animal or plant cell or a bacterial cell) and exploit its structures and enzymes of which they are lacking. Viruses that infect and exploit bacterial cells are called “bacteriophages.”

Viruses

Viruses (a term derived from the Latin word virus, meaning poison or toxin) are formed by a nucleic acid (RNA or DNA), surrounded by a protein, lipid or glycoprotein coating (ie consisting of a layer of proteins bound to sugars, which externally coat the virus), called capsid.

The envelope surrounding the viral genome has absolutely characteristic and very imaginative shapes for each type of virus, visible only under an electron microscope.

After making contact with a host cell, leaning on the outer surface of the cell membrane or bacterial cell wall, viruses insert their nucleic acid into the cytoplasm of the cell itself, assuming the direction of its metabolic functions.

The host cell, thus infected, continues to survive and reproduce, but produces mostly viral proteins and the genome of the virus instead of the usual products, necessary for its metabolism and replication. It is this process that earns viruses the nickname “parasites”.

The transmission of viruses from one individual to another can occur vertically (from mother to child, during pregnancy or at the time of delivery) or horizontally (from person to person, in different ways).

The ability of the virus to transmit depends on the type of virus. Some can be transmitted through simple contact, for example with saliva exchanges, while others are dispersed in the air by coughing or sneezing and, therefore, inhaled by another person present within a few meters. With the latter modality, the viruses responsible for flu, sore throats and colds are typically transmitted.

Other viruses to pass from one person to another require sexual contact, fecal-oral exchange (through contaminated food or water) or blood or other biological material (exchange of syringes, direct contact with infected blood, etc.). Finally, there are viruses that need a vector, such as an insect, to be transported from one person to another (as in the case of dengue virus and Zika virus, transmitted by infected mosquitoes).

The effects on humans caused by a viral infection can be extremely varied and characterized by very different levels of severity. The human pathologies caused by viruses are several and include, in addition to most of the seasonal ailments (colds, flu, pharyngitis, etc.). and gastroenteritis, various exanthematous diseases typical of childhood (measles, chickenpox, rubella etc.), many infections that can be contracted during trips to exotic places (yellow fever, dengue, ebola, chikungunya etc.), as well as hepatitis, papillomas, oral and genital herpes, poliomyelitis, rabies, HIV / AIDS, SARS, and, in the past, smallpox (eradicated worldwide since 1980 thanks to extensive vaccination campaigns).

When the body detects a viral infection, it begins to set up a fairly “generic” immune response (i.e. not specific to the particular virus present). In particular, a process is initiated, called “RNA interference”, which is crucial in the fight against viruses because it degrades the viral genome and allows cells to survive infection. Subsequently, the immune system produces antibodies specific, capable of binding to viruses and making them non-contagious. In addition, T lymphocytes (particular cells directly involved in the immune response to infections) are recruited en masse to destroy viruses.

Sometimes, unfortunately, these defense mechanisms cannot be exploited effectively because some viruses, such as HIV, specialize in evading the immune system using a number of different techniques, including that of continuously modifying their genome (through genetic mutations) and, therefore, their characteristics, making themselves unrecognizable and substantially unassailable from time to time.

The weapons available to counteract viral infections are not many. Effective antiviral drugs are only available for a few diseases (e.g. HIV/AIDS, hepatitis B and C, chickenpox, herpes simplex and partly influenza); In all other cases, it must be the immune response to eradicate the viruses, while the malaise present can be alleviated by supportive therapy with antipyretic/anti-inflammatory drugs, hydration, etc.

When a viral infection is present, moreover, to promote healing it is always essential to rest which, although not having a direct antiviral effect, allows the body to concentrate its energies in the fight against pathogens.

Some antiviral drugs help the body to completely eliminate the virus, and once healing has occurred, therapy can be stopped. Other drugs, on the other hand, while exerting an effective antiviral action, are not able to completely eliminate the pathogen, but only to inhibit its development and must therefore be taken regularly throughout life, if the disease is to be prevented from recurring (as in the case of HIV infection).

To prevent many viral infections, effective and safe vaccines are available, which should always be taken advantage of (except in the case of specific contraindications reported by the doctor) to protect yourself from avoidable illnesses and complications, as well as to reduce the amount of viruses present in the environment and, therefore, the general risk of infection at the population level.

The use of vaccines against a large number of viral infections for several decades has made it possible to significantly reduce the spread of serious and often lethal diseases, to the point of completely eradicating them in some regions of the world (for example measles in Mexico or polio in Europe) or, even, globally (as in the case of smallpox), saving millions of lives.

Bacteria

Unlike viruses, bacteria are real living beings according to the definition given by biology, i.e. organisms capable of living and reproducing autonomously, under adequate environmental conditions. They are neither plants nor animals, but belong to a group of their own and have very specific characteristics.

Key features include the fact of:

• be unicellular
• possess, in addition to the cell membrane, a second outer coating called “cell wall” (not present in animal cells)
• have within the cytoplasm all the organelles and substances necessary for its survival and multiplication, including the genome (always consisting of DNA), which is not enclosed in a cell nucleus (as happens in animal cells, for this reason called “eukaryotes”).

Bacteria are ubiquitous, meaning they are everywhere, and live in colonies, which can give rise to extremely varied “blooms” in shapes and colors when they are left to grow on culture medium in the laboratory, for experimental purposes or to arrive at the diagnosis of specific infections.

Some bacteria can survive in conditions that no other living organism could withstand (extreme temperatures, very high pressures, environments saturated with toxic substances), but the majority proliferate well in hot-humid environments, with temperatures between 10-15 ° and 40-45 ° C.

They are visible under the light microscope, both in solution and in blood, saliva or other biological fluids, and are mainly presented in three different forms:

• spherical (they are usually the simplest bacteria and are called “cocci”)
• stick (called “bacilli”, if they are straight, and “vibrios”, if they are curved)
• spiral (known as “spirilli” or, if the spiral is very narrow, “spirochete”).

Depending on the color they assume (or not) when treated with particular pigments (Gram coloring), they are distinguished:

• Gram positive bacteria (Gram+, appearing blue-violet under a microscope)
• Gram negative bacteria (Gram-, which appear pink under a microscope).

The first group includes useful microbes such as lactobacilli, but also pathogenic bacteria such as clostridia, enterococci, staphylococci, the causative agent of diphtheria (Corynaebacterium diphtheriae, which mainly affects the throat and airways), listeria (contracted by food and dangerous for newborns, the elderly and pregnant women) and Gardenella vaginalis (often responsible for vaginal infections).

Gram- bacteria comprise most microbes capable of causing disease in animals and humans, even severe or lethal if not treated promptly with a suitable antibiotic. The main ones are E. coli, salmonella, shigelle and other types of enterobacteria (all responsible for serious intestinal infections), klebsielle (which can infect both the intestine and the respiratory system), pseudomonas (cause of skin, respiratory, ocular, urinary infections, etc.), neisserie (including Neisseria meningitidis, one of the main causes of meningitis). bacterial), bacteria of the genus Proteus, Haemophilus and Yersinia (including the bacterium that causes the plague).

It should be emphasized that not all microbes are harmful. Many are even essential to our lives: the human body contains huge amounts of harmless and often useful bacteria, such as those present in the gastrointestinal system (microbiome), which help digestion and assimilation of nutrients and which support countless physiological functions, including the immune response.

The reproduction of all bacteria, pathogenic or harmless, generally takes place by simple division (or binary cleavage), during which there is first an increase in the size of the bacterium, which synthesizes and develops the various cellular structures, and then the splitting into two units, identical to the original.

The spread of pathogenic bacteria can occur in different ways, depending on the source from which the infection comes and the route of transmission. The sources can be “biological”, when they are represented by other individuals, animals or insects, or “environmental”, when the bacterium is present in contaminated water, food, surgical instruments, etc.

If, despite this initial attack, bacteria manage to multiply within tissues or blood, the immune system is further activated, producing specific antibodies against different types of bacteria and sending T lymphocytes to destroy them.

Once the infection has been defeated, in general, a small proportion of cells that have produced these antibodies is preserved and constitutes the “immunological memory” at the base of immunity against bacterial infections already experienced (or against which one has been vaccinated).

Unfortunately, the natural immune response can only eliminate the mildest bacterial infections or infections caused by less dangerous microbes, while it can do very little against aggressive bacteria with high replication capacity. To help the body eliminate these pathogens, it is essential to provide therapy with an antibiotic (or, in severe cases, even with combinations of different antibiotics).

An antibiotic therapy conducted correctly, respecting dosages, times and methods of intake indicated by the doctor, allows to eliminate the majority of common infections within 5-10 days. Before taking it, however, you must have a certain diagnosis of bacterial infection and, ideally, also have identified the microorganism responsible for being able to choose and use the antibiotic in a targeted way.

Over the years, antibiotic therapies, but above all vaccines and mass immunization programs have been able to eliminate, in industrialized countries, many of the most dangerous infectious diseases, such as leprosy, plague or diphtheria.

To conclude, it should be remembered that there are other unicellular microbes, other than bacteria, that live in colonies and that can be useful to humans or cause diseases of various types, more or less serious. These are fungi and yeasts (also called molds).

Among the first, the most famous are those that cause skin, nail and scalp mycoses, but there are also more fearsome ones responsible for blood infections that are very difficult to counteract.

Of the latter, the best known is Candida albicans, responsible for common benign but very annoying vaginal infections, as well as more demanding dermatitis and mucositis (infections associated with strong inflammation, respectively of the skin and mucous membrane of the mouth).

Other yeasts potentially harmful to humans are aspergilli (which constitute, for example, the classic bread mold and black mold from humidity that form in homes): these microorganisms are a frequent cause of allergic asthma in sensitive people and can give rise to very serious systemic infections in immunosuppressed and severely debilitated people (pulmonary / invasive aspergillosis).