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Introduction to Cells, DNA, and Viruses
Published in Patricia G. Melloy, Viruses and Society, 2023
Viruses typically contain a bit of nucleic acid (DNA or RNA) containing its small genome, have a protein coat, and may or may not be covered in an envelope or membrane made of lipids like the phospholipids that make up our own cell membrane. When the virus is fully assembled with its nucleic acid in its protective coat, it is called either a virion or a viral particle (Lostroh 2019). Assembly of new virions depends on the host cell’s raw materials to make the new nucleic acid, protein, and lipids, as well as the machinery for transcriptionof mRNA and translation of proteins. Viruses can typically exist in one of two states: lytic or lysogenic. Viruses may go through a period when they are in the cell and get passed from cell to cell as they divide, but the virus has actually inserted its genome into the host cell’s genome. The virus is said to be in a latent or lysogenic state. Some viruses may exist as “persistent infections” where virions are not actively being made for years (Lostroh 2019).
Introduction to virus structure, classification, replication, and hosts
Published in Avindra Nath, Joseph R. Berger, Clinical Neurovirology, 2020
Philippe Simon, Kevin M. Coombs
Some viruses will bypass the replicative cycle and instead will make use of an alternative pathway termed the lysogenic cycle. Among the herpesviruses, this is generally known as latency, but lysogeny refers to the general phenomenon, originally described for some bacterial viruses. In the lysogenic cycle, the viral DNA will undergo some alteration that results in the viral replicative cycle being arrested. In some cases (e.g., the bacterial virus lambda), the viral DNA integrates into the host cell chromosome. This effectively puts the viral replicative cycle “on hold,” because no progeny virions will be produced. Once integrated, the viral DNA can persist there indefinitely; each daughter cell will contain one or more copies of the viral DNA, because the incorporated DNA is replicated along with the cell’s DNA. As part of the host DNA, the viral DNA can remain silent, can serve to express a low copy number of genes, or can be induced to complete the replicative cycle. Exactly how induction is carried out in the host remains poorly understood, but stress, sunlight, other infections, and certain chemicals can function as inducers. The herpesviruses arrest their replicative cycles and enter latency by a different mechanism. In this case, the viral DNA circularizes and persists in the host cell’s nucleus, being passed to daughter cells, and may eventually be induced.
Physiology and Growth
Published in Paul Pumpens, Single-Stranded RNA Phages, 2020
In contrast to the lytic cycle of the phage reproduction, the lysogenic cycle assumes formation of a prophage that is either integration of the phage genome into the host bacterium's genome or formation of a circular replicon in the bacterial cytoplasm, while the host cell continues to live and reproduce normally. Although absence of the elements complementary to the RNA phage genome in the host's chromosome (Doi and Spiegelman 1962) and full independence of the phage replication from the DNA synthesis (Cooper and Zinder 1962; Knolle and Kaudewitz 1964) were established at the first steps of the RNA phage studies, the idea of the putative lysogeny persisted.
Bacteriophages for ESKAPE: role in pathogenicity and measures of control
Published in Expert Review of Anti-infective Therapy, 2021
Amrita Patil, Rajashri Banerji, Poonam Kanojiya, Santosh Koratkar, Sunil Saroj
Bacteriophages are viruses, which specifically infect bacterial cells and are one of the most abundant organisms in the environment (total population of ~1030–1032). Bacteriophage possesses tremendous diversity in morphology and genomic content enclosed in its capsid. The basal plate of the bacteriophage consists of tail fibers required for the initial attachment to the host cells. The bacteriophage genetic material is introduced to the host cell via the sheath region, often surrounded by sheath proteins [13]. Inside the host bacterial cell, bacteriophage undergoes either lytic (virulent) or lysogenic (non-virulent) life cycle. The bacteriophage is dependent on the host machinery for the synthesis of bacteriophage particles, assisted by early proteins synthesized during the lytic cycle. The genetic material is then packaged into the capsid constructing a new daughter bacteriophage that eventually bursts into the surrounding after lysis of the host bacterial cell. However, during the lysogenic cycle, rather than undergoing lysis, bacteriophage persists in the host in a dormant state. Bacteriophages and hosts in the lysogenic cycle are known as prophage and lysogen, respectively [14]. Based on the environmental cues, such as changes in pH, nutrients, temperature, and exposure to foreign DNA, hydrogen peroxide, antibiotics, or other agents causing DNA damage prophage enter into a lytic cycle and lead to lysis of the host cell [15] (Figure 1).
Merits of the ‘good’ viruses: the potential of virus-based therapeutics
Published in Expert Opinion on Biological Therapy, 2021
Qianyu Zhang, Wen Wu, Jinqiang Zhang, Xuefeng Xia
Phages play important roles in bacterial pathogenicity and evolution [10]. They can be categorized into obligate lytic (virulent) phages and lysogenic (temperate) phages. As for the anti-bacterial applications, lytic phages are more suitable than lysogenic phages as they are more similar to antibiotic drugs which induce rapid bacterial death [11]. Besides, the lysogenic conversion induced by lysogenic phages might lead to harmful consequences such as endowing bacteria with pathogenic traits or even antibiotic resistance [11]. In early years phages were investigated to manage conditions like typhoid fever, colitis, peritonitis, skin infections, surgical infections, septicemia, urinary tract infections, and otolaryngology infections [12]. However, data collected from early clinical studies lacked standardization such as blinding and the inclusion of appropriate control groups. The interest in phage therapy against bacterial infection was rekindled after Smith & Huggins devised pioneering studies against antibiotic-resistant bacterial strains in human and multiple animal models of septicemia and meningitis in the 1980s. There have been case reports on the antibacterial therapy using bacteriophages in recent years, and clinical trials have been carried out as well [13].
Gut non-bacterial microbiota contributing to alcohol-associated liver disease
Published in Gut Microbes, 2021
Wenkang Gao, Yixin Zhu, Jin Ye, Huikuan Chu
Fortunately, the Gut Virome Database (GVD)110 and the Gut Phage Database111 have been established and are continuously updating, which will advance research of gut virome considerably. According to the GVD,110 97.7% of viral populations are phages, 2.1% are eukaryotic viruses, and 0.1% are archaeal viruses. About 1015 bacteriophages settle in a healthy human intestine, which is 10 times the number of symbiotic bacteria.112 The main phages come from the order Caudovirales including Siphoviridae, Myoviridae, and Podoviridae.113,114 Some phages are highly specific to certain bacterial strains, while others have broader ranges of host cells.115 After infection of host by phages, highly virulent phages often cause cell lysis (lytic cycle) while mild phages either lyse the host cell or keep the host cell alive and reproduce normally (lysogenic cycle).116,117