Viruses The Interface Between Living and Nonliving

Viruses: Acellular Infectious Agents All viruses are obligate intracellular parasites: specific hosts Carry out no metabolic functions Replication inside a host cell only Very small colloidal particles: nm

Virus Structure Nucleic acid –May be either DNA or RNA –May be either single or double stranded Capsid –Outer protein coat surrounding nucleic acid –Composed of individual capsomeres Envelope –Only in some animal viruses (budding viruses) –Derived from host cell

Examples of Virus Structure

Tobacco Mosaic Virus

Pox Virus

Polio Virus

Phi X174

Structure of Bacteriophages

Viral Replication Lytic –Viral nucleic acid enters host –Takes over host cell metabolic machinery –Synthesis of new viral components –Assembly –Release of new viral particles Lysogenic –Viral nucleic acid enters host –Viral NA incorporates into host genome –Induction causes viral replication and initiates lytic replication

Viruses in Water and Wastewater

Detection/Enumeration Methods Plaque/Pock Assay –Inoculate viral suspension into susceptible host and count areas of cell death Direct counts via EM Detection of viral nucleic acid: PCR

Plaque Assay

PCR Polymerase chain reaction Amplification of a specific sequence of DNA Taq polymerase

Viruses in Water and Wastewater Over 140 types of enteric viruses may be found in fecally contaminated water –Hepatitis –Rotavirus –Norwalk type agent –Poliovirus –Coxsackievirus –Echovirus –Adenoviruses –Astroviruses –Caliciviruses

Transport of Viruses in the Subsurface

Treatment: Viruses