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There are bacteria that can enter body cells as parasites.

Could it be that some of these are benign, such that the guest will not kill the host cell it lives in (especially in human)?

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  • $\begingroup$ I think lactobacillus might be a good answer here $\endgroup$ – rhill45 Jan 27 '15 at 11:07
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    $\begingroup$ @ChrisStronks, thanks for the edition. Yes I tried to google it, didn't find good answer. And yes, it's really the direction of mitochondria, if there is something like that, at least not harmful to human. $\endgroup$ – Robertos Jan 27 '15 at 11:22
  • $\begingroup$ @Barbos, no worries. Interesting thoughts. +1 $\endgroup$ – AliceD Jan 27 '15 at 11:23
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    $\begingroup$ @rhill45 Lactobacillus don't enter cells IIRC. I think some Wolbachia infections of insects may be possible, not very sure though. $\endgroup$ – March Ho Jan 27 '15 at 11:29
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    $\begingroup$ @MarchHo, it is relevant, helpful and interesting! Thank you. Also I added "in human" a little bit after posting the question - so I need to say sorry. Just already heard before, that in some spices there are some symbiotic relationships with microbes (inside cells), but it's good to see real example. So again, thank you! $\endgroup$ – Robertos Jan 27 '15 at 13:02
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Survival of the host cell is in many cases dependent on the egress strategy of the pathogen. There are many examples documented in various species. The only case I found that's somewhat relevant to human cells was were mutant Legionella pneumophila, defective in the lytic pore complex, did not produce necrosis of the host cell. However, the cells did release the bacteria after a while, probably apoptosis due to accumulation.

EDIT: I found an article documenting Cryptococcus neoformans infection.

Cryptococcus neoformans causes disease in people with immune deficiencies such as AIDS. Upon infection, C. neoformans cells are ingested by macrophage immune cells, which provide a niche for survival and replication. After ingestion, macrophages can expel the fungi without causing harm to either cell type, a process named nonlytic exocytosis.

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I couldn't find any examples in humans (aside from mitochondria, if those count), but there are examples of varying degrees of intracellular endosymbiosis in insects, plants, and single-celled organisms.

An example is Ca. Carsonella ruddii, an endosymbiontic bacterium of psyllids (a family of plant-feeding insects). The bacterium lives within specialized host cells called bacteriocytes.

Many insects harbor intracellular bacteria that produce and excrete essential amino acids into the host cytosol. Although, in some cases, ‘dead-end’ bacterial cells with incomplete genomes are replaced by new endosymbionts, this is not the case for the bacteriocyte Carsonella ruddii. The genome of Carsonella is reduced to a mere 160 kb (within the range for organelles), has lost all genes for many essential functions, and no other bacterial endosymbionts are present to offer potential compensation. Thus, Carsonella seems to be at the end of an endosymbiotic continuum in sap-feeding psyllid insects and represents the recent acquisition of a new type of organelle for biosynthesis of amino acids.

Source: The intracellular cyanobacteria of Paulinella chromatophora: endosymbionts or organelles?

Wikipedia's article on endosymbionts has some other interesting examples.

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there is a word symbiosis which means interaction between two different organisms living in close physical association, typically to the advantage of both.Often, especially with microbes, one organism lives inside another — the host. symbiotic microbesWhen both organisms benefit from the relationship, it is called mutualism. When only one organism benefits, but the other one is not harmed, it is called commensalism. and yes,bacteria can have symbiotic relationship with human.bugs inside One example is the bacteria that live inside the human digestive system. These microbes break down food and produce vitamins that humans need. In return, the bacteria benefit from the stable environment inside the intestines. Bacteria also colonize human skin. The bacteria obtain nutrients from the surface of the skin, while providing people with protection against more dangerous microbes. moreover Your body is made up of around ten trillion cells, but you harbour a hundred trillion bacteria. For every gene in your genome, there are 100 bacterial ones. This is your ‘microbiome’ and it has a huge impact on your health, your ability to digest food and more. microbiome

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