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Competition between Chlamydia psittaci and L cells for host isoleucine pools: a limiting factor in chlamydial
Abstract:
L cells (mouse fibroblasts) supported the multiplication of the obligately intracellular parasitic bacterium Chlamydia psittaci (strain 6BC) when incubated in fresh growth medium (medium 199 + 5% fetal calf serum). When incubated in the medium supernatant from a 24-h-old culture of uninfected L cells (24-h used medium), uninfected cells did not divide and infected cells did not provide an adequate environment for the multiplication of C. psittaci, which persisted in a noninfectious latent state within the host cells. The failure of both L cells and chlamydiae to divide resulted from an overall reduction in the rate of protein synthesis by both host and parasite brought about by an insufficiency of the essential amino acid isoleucine in 24-h used medium. The concentration of isoleucine required to activate minimal growth of C. psittaci also minimally stimulated uninfected L cells to divide. The addition of cycloheximide to 24-h used medium also activated the latent chlamydial infection because it stimulated the incorporation of host protein-derived isoleucine into chlamydial protein. The results suggest that the chlamydial parasite and the L-cell host compete for the isoleucine in the soluble pool of the host cell and that the parasite is capable of sequestering isoleucine for its own biosynthetic needs only when the concentration of isoleucine in the host pool rises above the level required to maintain the hose in the stationary state. Extrapolation of the results obtained with the L cell-C. psittaci model system to natural latent chlamydial infections is discussed.
Insights
Chlamydia psittaci multiplication in L cells depends on isoleucine availability. Insufficient isoleucine causes latent infections, with the bacteria and host cells competing for this essential amino acid.
Area of Science:
- Cell biology
- Microbiology
- Parasitology
Background:
- L cells (mouse fibroblasts) support Chlamydia psittaci growth in fresh medium.
- Used medium from uninfected L cells inhibits both host cell division and bacterial multiplication.
- C. psittaci enters a latent state in used medium due to an inadequate host environment.
Purpose of the Study:
- To investigate the role of isoleucine in the host-parasite relationship between L cells and C. psittaci.
- To understand the mechanism behind latent chlamydial infections.
- To explore the competition for essential amino acids between host cells and intracellular bacteria.
Main Methods:
- Incubation of L cells infected with C. psittaci in fresh and used growth media.
- Analysis of protein synthesis rates in host cells and bacteria.
- Supplementation of used medium with isoleucine and cycloheximide to observe effects on growth and infection.
Main Results:
- Isoleucine insufficiency in used medium reduces protein synthesis in both L cells and C. psittaci, leading to host cell stasis and bacterial latency.
- The concentration of isoleucine required for minimal C. psittaci growth also minimally stimulates L cell division.
- Cycloheximide addition to used medium reactivates latent infections by facilitating isoleucine incorporation into bacterial proteins.
Conclusions:
- L cells and C. psittaci compete for isoleucine; the parasite sequesters it when host pool levels rise.
- Isoleucine availability is critical for C. psittaci replication and the transition from latent to active infection.
- Findings provide insights into natural latent chlamydial infections and host-pathogen nutrient competition.