Viruses and Bacteria


Viruses are parasites, typically ranging from 0.02 to 0.3 μm. They are completely dependent on cells (bacteria, plants or animals) to reproduce. Viruses have an outer covering of protein and sometimes lipids, a core of RNA or DNA, and some have the enzymes needed for the first steps of viral replication.

They are mainly classified according to the nature and structure of their genomes and their methods of replication. Thus, there are DNA viruses and RNA viruses. Each type can contain single or double stranded genetic material.

DNA viruses normally replicate in the nucleus of the host cell and RNA viruses in the cytoplasm.

For infection to occur, the virus first attacks the host cell at one of several receptor molecules on the cell surface. The viral DNA or RNA then separates from the outer layer (uncapsidation) and replicates inside the host cell, in a process that requires specific enzymes. The newly synthesized viral components then form a complete viral particle.

The host cell usually dies, releasing new viruses that infect other host cells. Each step of viral replication recruits different enzymes and substrates and provides an opportunity to interfere with the infection process.

The SARS-CoV-2 virus that causes COVID-19 is transmitted primarily through the droplets that are generated when an infected person coughs, sneezes, or exhales. These droplets are heavy and therefore do not remain suspended in the air for long periods of time, but deposit quickly on floors or surfaces.

Anyone can be infected by breathing in the virus if they are close to someone with COVID-19 or touch a contaminated surface and then touch their eyes, nose, or mouth.

SARS-CoV-2 is a ribonucleic acid (RNA) virus, whose genetic material is represented by a single positive RNA molecule (RNA+). It is classified as RNA+ because of its 5’3′ direction, which means that it can be read directly by the cellular structures of the host cell. It is considered a type of messenger RNA that induces the production of viral proteins. Another characteristic of this type of RNA is the presence of the enzyme RNA polymerase, which either accompanies the virus or is produced by the infected cell. This enzyme allows the production of a negative RNA molecule (RNA-) from the RNA+ molecule. This negative RNA molecule (RNA-) is transient, and from it, numerous positive RNA molecules (RNA+) are produced that are identical to the original positive RNA (RNA+). Therefore, the negative RNA molecule (RNA-) serves as a template for the production of positive RNA molecules (RNA+) that are descendants of the virus that infected the cell, and these descendants will parasitize the cell.

With the viral proteins already produced, assembly of the virions occurs by the inclusion of the positive RNA molecules (RNA+) in protein capsids. As they approach the cell plasma membrane, the virions are surrounded by a lipid bilayer, which opens at the end of the process, releasing a large amount of virus outside the cell.


Bacteria are single-celled organisms that have no defined nucleus and no membranous organelles. They can be classified according to their shape, the most common forms being spherical (cocci), rod-shaped (bacilli) and spiral-shaped (spirochetes or spirilli). They occur singly or in clusters. Bacterial cells have a cell wall, a structure located outside the plasma membrane.

They reproduce asexually by a process called binary division. Binary division occurs when a bacterium duplicates its genetic material and then divides, giving rise to two genetically identical daughter cells.

They are also classified into two groups due to a technique called Gram staining that allows the analysis of the differences in their cytological structure. Some, besides the plasma membrane and the cell wall (formed by peptideoglycans), have a third lipoprotein coating, containing lipopolysaccharides. When Gram stained, the cell wall of bacteria can be stained violet or red. Violet stained bacteria are called gram-positive bacteria and are characterized by the absence of the lipoprotein layer. Gram-negative bacteria, on the other hand, are colored red and have a more complex structure with a lipopolysaccharide layer.

Determining whether a bacterium is gram-positive or gram-negative is important in indicating which treatment should be adopted in the event of infections. Gram-negative bacteria are generally more tolerant and gram-positive bacteria are more sensitive to antibiotics.

Staphylococcus aureus

Staphylococcus aureus is a spherical bacterium from the group of gram-positive cocci that is part of the human microbiota, and is often found on the skin and in the nasal cavities of healthy people, but can cause diseases ranging from simple infections such as pimples and boils to more serious ones such as pneumonia and meningitis, among others. This bacterium was one of the first to be controlled with the discovery of antibiotics; however, due to its enormous capacity for adaptation and resistance, it has become one of the most important species in hospital and community infections.

The distribution of S. aureus is very wide, since this bacterium is significantly able to resist desiccation and cold and can remain viable for long periods of time in dust particles. It can be found in the environment, and humans are its main reservoir, being present in various parts of the body such as the nasal passages, throat, intestines and skin.

In the early 1970s, methicillin-resistant strains of S. aureus, identified by the acronym MRSA (Methicillin-resistant Staphylococcus aureus), and also resistant to other betalactams (cephalosporins and others) began to appear rapidly. MRSA strains have rapidly spread in hospital settings, thus limiting antibiotic treatment.

Combating antibiotic-resistant strains of bacteria is considerably difficult, and we must therefore take measures to reduce the risk of these strains developing.

Klebsiella pneumoniae

Klebsiella pneumoniae is a gram-negative, rod-shaped bacterium (bacillus), facultative aerobic, nonsporulating, and ranging in size from 0.3 to 1 µm in diameter.

Infections caused by Klebsiella spp. usually occur in people with a depressed immune system and are responsible for a high mortality rate. Among the most frequent clinical syndromes are pneumonia, urinary tract and wound infections, chronic atrophic rhinitis, arthritis, enteritis, meningitis in children, and sepsis (generalized infection).

In Gram-negative bacteria, the production of β-lactamases is the main form of bacterial resistance to β-lactam antimicrobials. β-lactamases are enzymes that promote degradation of the β-lactam ring, inactivating the antimicrobial and preventing its activity against enzymes responsible for bacterial cell wall synthesis.

Among the β-lactamases, the groups of most concern today are the extended aspect β-lactamases and the carbapenemases. The carbapenemases occur most frequently in enterobacteriaceae.

Enterobacteriaceae are a family of Gram negative bacteria, present in the intestines of humans and other animals. Carbapenem-resistant enterobacteriaceae (ERC) are a subgroup of this family, which have resistance to antimicrobials of the carbapenem group.

Carbapenemase-producing Klebsiella pneumoniae is one type of carbapenem-resistant bacteria. Klebsiella pneumoniae carbapenemase (KPC) is a β-lactamase. This enzyme confers resistance to all β-lactam agents (antibiotics) such as cephalosporins, penicillins, monobactams, and even carbapenems. This last class of antimicrobial is broad-spectrum and is often used to treat infections caused by multidrug-resistant bacteria. Thus, for the treatment of KPC-producing microorganisms, few therapeutic options remain. This characteristic, together with the fact that KPC has a high potential for spread, has become a cause for concern in hospitals and health care institutions worldwide.


Spores are small structures produced by bacteria, fungi, and plants, with the ability to generate a new individual. Because they are extremely small and light, spores can remain in the air for long periods of time and be moved over great distances.

Bacterial Spores

Bacterial spores or endospores act as survival structures when the bacterium finds itself in unfavorable environmental conditions. They are produced by the bacteria themselves and are found freely inside the bacteria.

Sporulation is the process by which bacteria produce spores and occurs through the invagination of a double layer of cell membrane, which closes to envelop a chromosome and a small amount of cytoplasm, thus ensuring the survival of the species. This layer is responsible for resistance to staining and to attack by the physical and chemical agents of sterilization and disinfection. In the spore stage, the bacterium can remain dormant for a long time, until conditions become favorable again. During this period, metabolism is reduced and multiplication and growth do not occur.

Spores are resistant to heat, dehydration, radiation, and pH changes, and can remain viable for centuries.

When the environmental conditions are favorable, the spore absorbs water until it swells and ruptures. Then germination occurs, producing a cell identical to the parental cell.

Fungal Spores

The sexual and asexual reproduction of fungi is ensured by the production of spores.

Asexual spores are formed by mitosis and subsequent cell division, without fusion of nuclei. When the spores germinate, they become organisms that are genetically identical to the parent. Sexed spores, on the other hand, result from the fusion of the nuclei of two opposing cross strains of the same fungal species. This type of spore is less frequent.

Dynasys – Engenharia e Telecomunicações, S.A.

Centro Empresarial Sado Internacional – Edifício E4

Estrada Nacional 10, Vale da Rosa

2910-835 Setúbal, Portugal

Tel: +351 265 706 900

Fax: +351 265 706 909