🦠Microbiology Unit 6 – Acellular Pathogens

Introduction to Acellular Pathogens

• Acellular pathogens are infectious agents that lack a cellular structure and consist of genetic material (DNA or RNA) encased in a protein coat
• Include viruses, viroids, and prions which are responsible for a wide range of diseases in humans, animals, and plants
• Differ from cellular pathogens (bacteria, fungi, and parasites) in their structure, replication mechanisms, and host interactions
• Rely on host cells for replication as they lack the necessary cellular machinery to reproduce independently
• Can infect various cell types and cause acute or chronic infections depending on the specific pathogen and host factors
• Have a significant impact on public health and require specialized diagnostic, treatment, and prevention strategies
• Continuously evolve through mutations and genetic recombination, leading to the emergence of new strains and challenges in disease control

Types of Acellular Pathogens

• Viruses are the most common type of acellular pathogens and consist of genetic material (DNA or RNA) surrounded by a protein capsid and sometimes an envelope
  • Can infect a wide range of hosts, including humans (influenza virus), animals (rabies virus), and plants (tobacco mosaic virus)
  • Classified based on their genetic material, capsid symmetry, presence of an envelope, and replication strategies
• Viroids are small, circular, single-stranded RNA molecules that lack a protein coat and primarily infect plants
  • Cause diseases such as potato spindle tuber viroid and coconut cadang-cadang viroid
  • Replicate using host cell enzymes and can interfere with plant growth and development
• Prions are infectious protein particles that lack nucleic acids and cause progressive neurodegenerative disorders
  • Composed of misfolded proteins (PrPSc) that can induce the misfolding of normal cellular proteins (PrPC)
  • Associated with diseases such as Creutzfeldt-Jakob disease (CJD) in humans, bovine spongiform encephalopathy (BSE) in cattle, and chronic wasting disease (CWD) in deer and elk

Structure and Composition

• Viruses have a simple structure consisting of genetic material (DNA or RNA) encased in a protein capsid
  • Capsid is composed of multiple copies of one or a few types of proteins arranged in a symmetric manner (icosahedral, helical, or complex)
  • Some viruses have an additional lipid envelope derived from the host cell membrane, which may contain viral glycoproteins
• Viral genomes can be single-stranded or double-stranded, linear or circular, and range in size from a few thousand to millions of nucleotides
  • Genome can be segmented (influenza virus) or non-segmented (measles virus) and may have positive-sense, negative-sense, or ambisense orientation
• Viroids have a compact, circular, single-stranded RNA structure with extensive intramolecular base pairing
  • Lack a protein coat and range in size from 246 to 401 nucleotides
  • Contain self-cleaving ribozymes that facilitate their replication and processing
• Prions are composed of misfolded proteins (PrPSc) that can induce the misfolding of normal cellular proteins (PrPC)
  • PrPSc has a higher proportion of β-sheet structure compared to the predominantly α-helical structure of PrPC
  • Misfolded proteins aggregate and form amyloid fibrils, which accumulate in the brain and cause neurodegeneration

Replication Cycles

• Viruses undergo a series of steps to replicate within host cells, including attachment, entry, uncoating, genome replication, protein synthesis, assembly, and release
  • Attachment involves specific interactions between viral surface proteins and host cell receptors
  • Entry can occur through direct fusion with the cell membrane (enveloped viruses) or receptor-mediated endocytosis (non-enveloped viruses)
  • Uncoating releases the viral genome into the host cell cytoplasm or nucleus, depending on the virus type
  • Genome replication and protein synthesis utilize host cell machinery and vary based on the virus type (DNA or RNA, positive-sense or negative-sense)
  • Assembly involves the packaging of newly synthesized viral genomes and proteins into progeny virions
  • Release occurs through cell lysis (non-enveloped viruses) or budding from the cell membrane (enveloped viruses)
• Viroids replicate through a rolling circle mechanism using host cell enzymes
  • (+) strand viroid RNA serves as a template for (-) strand synthesis, which then acts as a template for multiple copies of (+) strand RNA
  • Newly synthesized (+) strand RNA undergoes self-cleavage and circularization to form mature viroid particles
• Prions propagate by inducing the misfolding of normal cellular proteins (PrPC) into the pathogenic form (PrPSc)
  • PrPSc acts as a template for the conversion of PrPC, leading to an exponential increase in misfolded proteins
  • Misfolded proteins aggregate and form amyloid fibrils, which accumulate in the brain and cause neurodegeneration

Transmission and Infection Mechanisms

• Acellular pathogens can be transmitted through various routes, including direct contact, respiratory droplets, fomites, vectors, and contaminated food or water
  • Direct contact transmission occurs through physical contact with an infected individual or their bodily fluids (HIV, herpes simplex virus)
  • Respiratory droplet transmission involves the inhalation of virus-containing droplets expelled by an infected individual through coughing, sneezing, or talking (influenza virus, SARS-CoV-2)
  • Fomite transmission occurs when a susceptible individual comes into contact with contaminated surfaces or objects (norovirus, rhinovirus)
  • Vector-borne transmission involves the transfer of the pathogen by an arthropod vector, such as mosquitoes (dengue virus, Zika virus) or ticks (tick-borne encephalitis virus)
  • Foodborne and waterborne transmission occur through the consumption of contaminated food or water (hepatitis A virus, norovirus)
• Viruses and viroids can infect host cells through specific interactions between viral surface proteins and host cell receptors
  • Viral entry can occur through direct fusion with the cell membrane (enveloped viruses) or receptor-mediated endocytosis (non-enveloped viruses)
  • Viroid entry into plant cells is not well understood but may involve mechanical damage or vector-mediated transmission
• Prions can be transmitted through the ingestion of contaminated tissue (BSE, kuru) or iatrogenic routes, such as contaminated surgical instruments or blood transfusions (variant CJD)
  • Prions can also be transmitted through genetic inheritance of mutations in the PRNP gene (familial CJD, fatal familial insomnia)

Pathogenesis and Disease

• Acellular pathogens cause disease through various mechanisms, including direct cell damage, immune system evasion, and host cell manipulation
  • Viruses can cause direct cell damage through lysis or apoptosis, leading to tissue injury and organ dysfunction (influenza virus, hepatitis B virus)
  • Some viruses can evade or suppress the host immune response, allowing for persistent infection and chronic disease (HIV, hepatitis C virus)
  • Viruses can manipulate host cell signaling pathways and gene expression to promote viral replication and spread (Epstein-Barr virus, human papillomavirus)
• Viroids cause disease in plants by interfering with gene expression and regulatory processes
  • Viroid infection can lead to stunted growth, leaf distortion, fruit malformation, and reduced crop yields
  • The exact mechanisms of viroid pathogenesis are not fully understood but may involve RNA silencing and disruption of host cell functions
• Prions cause progressive neurodegenerative disorders characterized by the accumulation of misfolded proteins in the brain
  • Misfolded prion proteins (PrPSc) aggregate and form amyloid fibrils, which lead to neuronal dysfunction and death
  • Prion diseases are characterized by long incubation periods, rapid progression, and invariably fatal outcomes
  • Examples include Creutzfeldt-Jakob disease (CJD) in humans, bovine spongiform encephalopathy (BSE) in cattle, and chronic wasting disease (CWD) in deer and elk

Detection and Diagnosis

• Acellular pathogens can be detected and diagnosed using various laboratory techniques, including serological tests, molecular assays, and microscopy
  • Serological tests detect antibodies produced by the host immune system in response to the pathogen (ELISA, Western blot, hemagglutination inhibition assay)
  • Molecular assays detect the presence of the pathogen's genetic material (PCR, RT-PCR, DNA sequencing)
  • Microscopy techniques, such as electron microscopy, can visualize the morphology and structure of viruses and prions
• Virus isolation and culture can be performed to detect and characterize viruses in clinical samples
  • Viruses can be propagated in cell cultures, embryonated eggs, or animal models
  • Cytopathic effects (CPE) and plaque assays can be used to quantify viral titers and assess infectivity
• Viroid detection primarily relies on molecular techniques, such as RT-PCR and RNA hybridization
  • Viroid RNA can be extracted from infected plant tissues and amplified using specific primers
  • Hybridization assays using labeled probes can detect the presence of viroid RNA in plant extracts
• Prion diseases are diagnosed based on a combination of clinical presentation, neuropathological findings, and laboratory tests
  • Neuropathological examination of brain tissue can reveal the presence of spongiform changes and amyloid plaques
  • Immunohistochemistry and Western blot can detect the presence of misfolded prion proteins (PrPSc) in brain tissue
  • Genetic testing can identify mutations in the PRNP gene associated with familial prion diseases

Treatment and Prevention Strategies

• Treatment options for acellular pathogens are limited and vary depending on the specific pathogen and disease
  • Antiviral drugs can be used to treat some viral infections by targeting specific stages of the viral replication cycle (oseltamivir for influenza, acyclovir for herpes simplex virus)
  • Combination antiretroviral therapy (cART) is used to manage HIV infection by suppressing viral replication and preserving immune function
  • Supportive care and management of symptoms are the primary treatment approaches for most viral infections and prion diseases
• Prevention strategies for acellular pathogens include vaccination, infection control measures, and public health interventions
  • Vaccines can provide protection against specific viral infections by stimulating the host immune response (influenza vaccine, measles-mumps-rubella vaccine)
  • Infection control measures, such as hand hygiene, respiratory etiquette, and isolation of infected individuals, can reduce the spread of viral infections in healthcare settings and the community
  • Public health interventions, such as surveillance, contact tracing, and quarantine, can help control the spread of emerging viral infections (SARS-CoV-2, Ebola virus)
• Prevention of viroid infections in plants relies on the use of virus-free planting materials, quarantine measures, and the eradication of infected plants
  • Viroid-free planting materials can be obtained through meristem culture and heat treatment of infected plants
  • Quarantine measures and strict biosecurity protocols can prevent the introduction and spread of viroids in agricultural settings
• Prevention of prion diseases focuses on reducing the risk of exposure to infectious prions
  • Regulations on the feeding of ruminant-derived proteins to other ruminants have been implemented to prevent the spread of BSE
  • Strict infection control measures and the use of disposable surgical instruments can reduce the risk of iatrogenic transmission of prion diseases

Ecological and Evolutionary Aspects

• Acellular pathogens have complex ecological interactions with their hosts and the environment
  • Viruses can infect a wide range of hosts, including humans, animals, plants, and microorganisms, and play important roles in shaping ecosystem dynamics
  • Viruses can transfer genetic material between different species through horizontal gene transfer, contributing to the evolution of their hosts
  • Viroids are plant pathogens that can have significant impacts on agricultural ecosystems and crop production
  • Prions can persist in the environment and have the potential to affect wildlife populations, as seen with chronic wasting disease in cervids
• Acellular pathogens evolve rapidly due to their high mutation rates, short generation times, and large population sizes
  • Viral evolution is driven by mutations, recombination, and reassortment events, which can lead to the emergence of new strains with altered virulence, transmissibility, or antiviral resistance
  • RNA viruses, such as influenza virus and HIV, have particularly high mutation rates due to the error-prone nature of their RNA-dependent RNA polymerases
  • Prions can undergo conformational changes and adapt to new host species, leading to the emergence of new prion strains with different pathogenic properties
• Acellular pathogens have co-evolved with their hosts over millions of years, leading to the development of complex host-pathogen interactions
  • Some viruses have evolved strategies to evade or suppress the host immune response, allowing for persistent infection and long-term survival
  • Hosts have evolved innate and adaptive immune mechanisms to detect and combat viral infections, driving the evolution of viral immune evasion strategies
  • The evolutionary arms race between acellular pathogens and their hosts has shaped the diversity and distribution of these pathogens in nature