Principles of Virology, Volume 1. Jane Flint

the linear nature of the dose-response curve indicates that a single particle is capable of initiating an infection (one-hit kinetics) (Fig. 2.8), the high particle-to-PFU ratio of many viruses demonstrates that not all virus particles are successful. High values are sometimes caused by the presence of noninfectious particles with genomes that harbor lethal mutations or that have been damaged during growth or purification (defective particles). An alternative explanation is that although all viruses in a preparation are in fact capable of initiating infection, not all of them succeed because of the complexity of the infectious cycle. Failure at any one step in the cycle prevents completion. In this case, a high particle-to-PFU ratio indicates not that most particles are defective but, rather, that they failed to complete the infection.

Table 2.1 Particle-to-PFU ratios of some animal viruses

Virus	Particle/PFU ratio
Papillomaviridae
Papillomavirus	10,000
Picornaviridae
Poliovirus	30–1,000
Herpesviridae
Herpes simplex virus	50–200
Polyomaviridae
Polyomavirus	38–50
Simian virus 40	100–200
Adenoviridae	20–100
Poxviridae	1–100
Orthomyxoviridae
Influenza virus	20–50
Reoviridae
Reovirus	10
Alphaviridae
Semliki Forest virus	1–2

Measurement of Virus Particles

      Although the numbers of virus particles and infectious units are often not equal, assays for particle number are frequently used to approximate the number of infectious particles present in a sample. For example, assuming that the ratio of infectious units to physical particles is constant, the concentration of viral DNA or protein can be used to estimate the number of infectious particles. Biochemical or physical assays are usually more rapid and easier to carry out than those for infectivity, which may be slow, cumbersome, or impossible. Assays for subviral components also provide information on particle number if the amount of these components in each virus particle is known.

       Electron Microscopy

      With few exceptions, virus particles are too small to be observed directly by light microscopy. However, they can be seen readily in the electron microscope. If a sample contains only one type of virus, the particle count can be determined. A virus preparation is mixed with a known concentration of latex beads, and the numbers of virus particles and beads are then counted, allowing the concentration of the virus particles in the sample to be determined by comparison.

       Hemagglutination

Members of the Adenoviridae, Orthomyxoviridae, and Paramyxoviridae, among others, contain proteins that bind to erythrocytes (red blood cells); these viruses can link multiple cells, resulting in formation of a lattice. This property is called hemagglutination. For example, influenza viruses contain an envelope glycoprotein called hemagglutinin (HA), which binds to N-acetylneuraminic acid-containing glycoproteins on erythrocytes. In practice, 2-fold serial dilutions of the virus stock are prepared, mixed with a known quantity of red blood cells, and added to small wells in a plastic tray (Fig. 2.10). Unlinked red blood cells tumble to the bottom of the well and form a sharp dot or button. In contrast, agglutinated red blood cells form a diffuse lattice that coats the well. Because the assay is rapid (30 min), it is often used as a quick indicator of the relative quantities of virus particles. However, it is not sufficiently sensitive to detect small numbers of particles.

       Centrifugation

      The use of centrifugal force to separate particles from solution according to size, shape, or density has been a staple of virology. The instrument used for such separations is called a centrifuge, which can range from small tabletop devices that accommodate small tubes to large floor models with greater capacity and to ultracentrifuges that can achieve revolutions per minute in excess of 70,000. The ultracentrifuge was invented by Theodor Svedberg in 1925, and it is the first initial of his last name that is used to describe the sedimentation coefficient of a particle as measured by centrifugation, e.g., the 16S ribosomal subunit.

Figure 2.10 Hemagglutination assay. (Top) Samples of different influenza viruses were diluted, and a portion of each dilution was mixed with a suspension of chicken red blood cells and added to the wells. After 30 min at 4°C, the wells were photographed. Sample A does not contain virus. Sample B causes hemagglutination until a dilution of 1:512 and therefore has a hemagglutination titer of 512. Elution of the virus from red blood cells at the 1:4 dilution is caused by neuraminidase in the virus particle. This enzyme cleaves N-acetylneuraminic acid from glycoprotein receptors and elutes bound viruses from red blood cells. (Bottom) Schematic illustration of hemagglutination of red blood cells by influenza virus. Top, Courtesy of C. Basler and P. Palese, Mount Sinai School of Medicine of the City University of New York.

It would not be wrong to state that every virology laboratory is in possession of at least one centrifuge and probably has access to more. The uses of the centrifuge in virology are manifold: from low-speed separation of virus particles from infected cell debris in cell culture medium to fractionation of infected cells to isolate nuclei, cytoplasm, or ribosomes, and to purification of virus particles.

Differential centrifugation is used to separate viruses, organelles, or subcellular structures from cells. Preformed gradients of sucrose are often used because particles that move with various velocities can be separated differentially in the increasing viscosity of the solution. One application of sucrose gradients is the purification of virus particles. Another is polysome profiling, an analysis of the mRNAs associated with ribosomes (Fig.