The effect of the partition locus on plasmid stability and expression of a prolonged chemostat culture

Investigation of subpopulation heterogeneity and plasmid stability in recombinant escherichia coli via a simple segregated model

Many microbial and cell cultures exhibit phenomena that can best be described using a segregated modeling approach. Heterogeneties are more marked in recombinant cell cultures because subpopulations, which often exhibit different growth and productivity characteristics, are more easily identified by selective markers. A simple segregated mathematical model that simulates the growth of recombinant Escherichia coli cells is developed. Subpopulations of different growth rate, plasmid replication rate, and plasmid segregation probability are explicitly considered. Results indicate that a third mechanism of plasmid instability, referred to here as a "downward selective pressure," is significant when describing plasmid loss in batch and chemostat cultures. Also, the model agrees well with experimental data from cultures under antibiotic selective pressure. Finally, model simulations of chemostat cultures reveal the importance of initial conditions on culture stability and the possible presence of nonrandom partitioning functions.

A host/plasmid system that is not dependent on antibiotics and antibiotic resistance genes for stable plasmid maintenance in Escherichia coli

Uneven distribution of plasmid-based expression vectors to daughter cells during bacterial cell division results in an increasing proportion of plasmid free cells during growth. This is a major industrial problem leading to reduction of product yields and increased production costs during large-scale cultivation of vector-carrying bacteria. For this reason, a selection must be provided that kills the plasmid free cells. The most conventional method to obtain this desired selection is to insert some gene for antibiotic resistance in the plasmid and then grow the bacteria in the presence of the corresponding antibiotic. We describe here a host/plasmid Escherichia coli system with a totally stable plasmid that can be maintained without the use of antibiotic selection. The plasmid is maintained, since it carries the small essential gene infA (coding for translation initiation factor 1, IF1) in an E. coli strain that has been deleted for its chromosomal infA gene. As a result only plasmid carrying cells can grow, making the strain totally dependent on the maintenance of the plasmid. A selection based on antibiotics is thus not necessary during cultivation, and no antibiotic-resistance genes are present neither in the final strain nor in the final plasmid. Plasmid-free cells do not accumulate even after an extended period of continuous growth. Growth rates of the control and the plasmid harboring strains are indistinguishable from each other in both LB and defined media. The indicated approach can be used to modify existing production strains and plasmids to the described concept. The infA based plasmid stability system should eliminate industrial cultivation problems caused by the loss of expression vector and use of antibiotics in the cultivation medium. Also environmental problems caused by release of antibiotics and antibiotic resistance genes, that potentially can give horizontal gene transfer between bacterial populations, are eliminated.

Studies on plasmid stability and LTB production by recombinant Vibrio cholerae in batch and chemostat cultures: a lesson for optimizing conditions for chemical induction

Plasmid content, its stability and the expression of B-subunit of Escherichia coli heat-labile enterotoxin (LTB) in Vibrio cholerae/r-pMMB68 system have been studied in batch as well as in chemostat cultures. Upon induction with isopropyl-beta-D-thiogalactopyranoside (IPTG), cultures secreted LTB into the extracellular milieu. Highest specific LTB production rate of 7.3 mg mg-1h-1 was achieved in batch culture induced at the late exponential growth phase. The plasmid pMMB68 was fairly stable up to 20 generations, even in the absence of selection pressure. Instability of the plasmid was accelerated in the presence of IPTG and at higher dilution rates. Maximum productivity of 2.1 mg l-1 h-1 was achieved in continuous culture, which remained constant at a range of dilution rates from 0.20 to 0.35 h-1.

Enhanced production of pL-controlled recombinant proteins and plasmid stability in Escherichia coli RecA+ strains

Overexpression of pL-controlled foot-and-mouth disease virus recombinant proteins was studied in Escherichia coli RecA+ strains and in a recA mutant. Higher protein yield and extractable plasmid DNA amounts were found in wild type cells, in absence of detectable RecA proteolytic activity. Minor but still significant differences in pBR322 DNA amounts were also detected between RecA+ and its recA13 and lexA1 derivatives. These data should be seriously considered to select expression systems and to design production processes for recombinant proteins, specially if they are expected to be toxic for Escherichia coli cells.