Rickettsia and viruses exhibit a high degree of dependence on the host. Rickettsia have bacterial cell structures but can only exhibit limited metabolic activity outside of host cells. Viruses are obligate intracellular parasites that can only multiply within host cells and have no independent metabolism, instead parasitizing host cell processes to synthesize new virus elements.

1. In the contextof communicable disease,the host-parasite relationshipmustbe considerednotonlywith
respectto the individual host-parasiteinteractionbutalsointermsof the interrelationshipbetweenthe
hostand parasite populations,aswell as those of anyotherhost speciesinvolved.Mostpathogenic
bacteriaare obligate parasites;thatis,theyare foundonlyinassociationwiththeirhosts.Some,suchas
staphylococci andstreptococci,canproliferate outside the bodyof the hostinnutritive materials
infectedfromhostsources.Withinthe tissuesof the host,these organismssetuplocal infectionsthat
spreadthroughoutthe body.Still otherbacteria,suchasthe glandersbacillus(Burkholderiamallei) and
the gonococci,meningococci,andpneumococci,are more closelyadaptedparasites,capableof
multiplyingoutside the bodyof the hostonlyunderthe artificial conditionsof the laboratory.All these
microorganismshave complete cell structuresandmetaboliccapabilities.
A greaterdegree of dependence onthe hostisshownbyrickettsiae andviruses.Rickettsiae are
microorganismsthathave the cell structure of bacteria.Theyexhibitasmall degree of metabolicactivity
outside cells,buttheycannotgrowinthe absence of hosttissue.The ultimate inparasitism,however,is
that of the viruses,whichhave noconventional cell structure andconsistonlyof anucleicacid(either
DNA or RNA) wrappedina protective proteincoat.Virusesare obligatoryintracellularparasites,capable
of multiplyingonlywithinthe cellsof the host,andtheyhave noindependentmetabolicactivityof their
own.The geneticinformationthatdirectsthe synthesisof virusmaterialsandcertainenzymesenters
the host cell,parasitizesitschemical processes,and directsthemtowardthe synthesisof new virus
elements.
Bacteria as parasite
Some bacteriaare obligate parasitesandgrow onlywithinalivinghostcell. RickettsiaandChlamydia,
for example,growineukaryoticcells,andBdellovibriogrow inbacterial cells.Treponemapallidumis
difficult,if notimpossible,togrowinculture,probablybecause itrequireslow oxygentensionandlow
oxidation-reductionlevels,whichare providedbythe presence of animal cells,ratherthananyspecific
nutrient.Because some bacteriamaythrive onlyasanimal orplantparasitesoronlyina rich source of
nutrientssuchas milk,theylikelydonotthrive asfree bacteriainnature.Manybacteriafrom natural
environmentsexistinaconsortiumwithotherbacteriaandare difficulttoisolate andculture separately
fromthe othermembersof thatpartnership.
Bacteria as host
The main functionof host E. coli cellsinbaculoviral expressionsystemistoconstructinfectiousbacmid
by Tn7 translocationrecombination.Inaddition,bacterialhostcellscanalsobe usedto complete some
difficultsubcloningconstructionandimprovethe copynumberandtransfectionefficiencyof
recombinantvector.The selectionof host,hostbacteriaisveryimportantforbacmid’sinfectiousand
infectionefficiency.Highqualityof hostbacteriaisone of the basic conditions forsuccessful protein
expression.Forcustomerswhowanttoconduct the proteinexpressionintheirownLab,Creative Bio
mart provideshighqualityhostbacteriatofacilitiesthe high-qualityvector/bacmidconstruction.As
expertsinthe fieldof biotechnology,ourproductshave provedtobe reliable.

2. Rickettsia and Chlamydia