PLAQUE ASSAY FOR PSITTACOSIS VIRUS IN MONOLAYERS OF CHICK EMBRYO FIBROBLASTS

Use of the plaque assay for testing the antibiotic susceptibility of intracellular bacteria

The plaque assay was first described for titration of bacterial inoculums and clonal isolation, and was later adapted for testing antibiotics susceptibility and to study virulence factors and motility of bacteria. Over time, the sensitivity and reproducibility of the technique has been improved. Usually, the number of plaques is counted; however, the recent development of informatics tools has stimulated interest in the quantification of plaque size. Owing to this new approach, the plaque assay has been used to characterize the host cell response when infected cells are treated with antimicrobial agents. It was found that statins prevented cell lesions following rickettsial infection; in other studies, some antibiotics were found to cause apoptosis of host cells, suggesting a toxic activity. Here, we present an overview of the plaque assay as it has been used to investigate intracellular bacteria.

Plaque formation by Chlamydia in L cells

Chlamydiae were found capable of producing plaques in several cell lines. Mouse fibroblast cells, L-929, proved the most sensitive to infection and yielded plaques of the highest clarity. Assay of chlamydial infectivity by plaque titration was at least as sensitive as egg ld(50) determination. Among chlamydial isolates of avian, mammalian, and human origin, only slow-growing trachoma-inclusion-conjunctivitis agents did not produce plaques. The plaque assay is highly sensitive, reproducible, and offers a potential tool for investigations requiring accurate measurement of small changes in chlamydial infectivity.

Plaque formation by and plaque cloning of Chlamydia trachomatis biovar trachoma

A new technique for the induction of plaque formation by Chlamydia trachomatis biovar trachoma applicable to the titration of infectivity and cloning of biovar trachoma was established. Three novel strains were cloned and confirmed to be free of glycogen inclusions. The lack of glycogen accumulation correlated with the absence of a 7.5-kb plasmid, which is highly conserved in other strains of C. trachomatis. Although the growth efficiency of these plasmid-free strains was slightly lower than that of plasmid-positive strains, possession of the plasmid and glycogen accumulation were not essential for the survival of C. trachomatis.

FLUORESCENT CELL-COUNTING NEUTRALIZATION TEST FOR PSITTACOSIS

Hahon, Nicholas (Fort Detrick, Frederick, Md.), and Kenneth O. Cooke. Fluorescent cell-counting neutralization test for psittacosis. J. Bacteriol. 89:1465-1471. 1965.-A sensitive, precise, and specific serological procedure, the fluorescent cell-counting neutralization test, was developed to detect and to measure quantitatively psittacosis serum-neutralizing antibodies within 24 hr. The test is based on the reduction of fluorescent cells in McCoy cell monolayers resulting from the neutralization of infective agent particles by specific antiserum. Small but significant rises in neutralizing titers were measured in serum specimens from monkeys previously exposed to the psittacosis agent and from humans with diagnoses of subclinical or established psittacosis infections.

Production and partial characterization of monoclonal antibodies to four Chlamydia psittaci isolates

Monoclonal antibodies (MAbs) were produced to four Chlamydia psittaci isolates: NJ1 and TT3 from turkeys, VS1 from a parakeet, and B577 from an ovine abortion. MAbs were tested for reactivity with each isolate by the indirect immunofluorescent antibody technique and for neutralization by an inclusion reduction neutralization technique in tissue culture. Two genus-specific and 14 serovar-specific MAbs were produced. Genus-specific MAbs reacted with all avian and mammalian isolates; however, each failed to neutralize its homologous chlamydial isolate. Turkey isolates NJ1 and TT3 were antigenically similar; serovar-specific MAbs produced to each reacted equally with both isolates yet showed little or no reaction with other serovars. Serovar-specific MAbs to the parakeet and abortion isolates were distinct; no cross-reactions were seen with other serovars. None of the serovar-specific MAbs reacted with an ovine arthritis isolate. Of the 14 serovar-specific MAbs, 13 partially neutralized homologous strains with or without the addition of complement. Because of the high specificity, the serovar-specific MAbs used with the immunofluorescence technique provided a rapid and precise method to identify three serovars of C. psittaci.

Plaque formation in chick embryo fibroblast cells by Chlamydia isolated from avian and mammalian sources

Fifteen strains of chlamydial agents, 2 Chlamydia trachomatis and 13 Chlamydia psittaci, were tested for plaque formation on chick embryo fibroblast cells. C. trachomatis strains did not form plaques; 12 strains of C. psittaci did form plaques and 1 strain did not. The distribution of plaque sizes with C. psittaci strains was studied and it was found that the 12 strains could be tentatively grouped into three plaquing types: small (<0.50 mm), intermediate (0.50 to 0.90 mm), and large (>0.90 mm). Small plaques were predominantly associated with strains M56, ET, CP3, CT4, EBA, CS1, and CT2. Intermediate size plaques were predominantly associated with strains NJ1, VT1, and H81. Large plaques were predominantly associated with strains WC and OG. Plaque size appears to be related to virulence of Chlamydia for laboratory animals.