© 2003 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume IIA: Biocompatibility, Landes Bioscience, Georgetown, TX, 2003

15.6.3.5 Intracellular Microbiota

Inside living cells dwell a number cell-like objects of variable volumetric occupancy. Most notably, mitochondria (Section 8.5.3.10) are energy-producing organelles present in virtually all eukaryotic cells that may vary in number throughout the life cycle of the cell. The volume of mitochondria per cell (typically 5-20%) increases in porcine pinealocytes for animals kept in continuous darkness and decreases for animals kept in continuous light [4569]. Mechanical cell injury can cause dramatic mitochondrial enlargement [3757]. Adrenergic innervation of cultured cardiac myocytes over a 96 hour period causes mitochondrial volume to rise +43% (from 521 micron³ to 744 micron³) although total cell volume also increases +44% (from 3344 micron³ to 4816 micron³), holding volume fraction constant at ~15% [4570]. Myocytes cultured on laminin have a higher mitochondrion count than cells grown on plastic [4571]. Resistance training increases total mitochondrial volume by up to +33% per cell [4572], and muscle overuse also elicits changes in mitochondrial count [4573]. The outer hair cells of the guinea pig cochlea have 1425 mitochondria/cell in the first row but 1963 mitochondria/cell in the third row (which has 2-3 times more nerve endings nearby) [4574], a +38% increase in the count.

Other biota that may live inside of cells include a variety of endosymbionts* [4575-4577] (of which the mitochondrion [4578] and cell nucleus [4579] are possible ancient examples). For instance, endosymbiotic bacteria can infect Amoeba proteus, quickly reaching the maximum carrying number of 42,000 organisms [4580]. Taking the volume of the bacterium and the amoeba as ~1 micron³ and ~10⁸ micron³, respectively, the volume fraction occupied by the endosymbionts is only ~0.04%.

* Interestingly, one rickettsial bacterial species called Wolbachia is thought to infect the reproductive tissues of as many as 20% of all insect species [4581, 4582]. This endosymbiont enhances its own transmission by establishing an active cytoplasmic incompatibility [4583] between egg and sperm cells of host strains or species, e.g., by inducing abortive karyogamy when an uninfected female mates with an infected male.

Individual lymphocytes (~200 micron³ [4584]) have been observed circulating for hours inside larger living cells (~3-5% volume fraction) with no evident ill effect, a phenomenon originally called emperipolesis (Section 8.5.3.12). Emperipolesis today refers to the temporary presence of one cell within another’s cytoplasm and has been associated with tumor cells [5985-5987], muscle cells [5984], megakaryocytes [5987-5991], thymic epithelial cells [5994] (nurse cells [6078]), human fetal liver Kupffer cells [5995], myeloproliferative disorders [5990-5993], and both cutaneous [5996-5998] and noncutaneous [5999] Rosai-Dorfman disease – though R. Bradbury notes that emperipolesis may not be a general property of mammalian cells. While neutrophils and macrophages are both found in mammalian lungs and neither cell regularly phagocytoses the other in significant quantities, alveolar macrophages containing neutrophils have been reported [763]. Neutrophils that have undergone apoptosis are taken up by macrophages, with a mean uptake of 3 neutrophils per macrophage [648]. Taking nonmigratory human neutrophils as 204 micron³ [4534] and human alveolar macrophages as 4990 micron³ [4562], this uptake represents ~12% of macrophage cell volume. It is unknown whether 12% remains a reasonable limit if the entire population of phagocytic cells in a tissue is burdened by that much foreign material, or if such burdens are tolerable only when a relatively few cells in the population are affected. The first case, analogous to an aggressive bacterial infection, has major implications for the entire multicellular organization, whereas the second case, analogous to emperipolesis, has only a minor volumetric impact.

Cells may also harbor smaller pathogens which are usually volumetrically harmless to the host. Perhaps the best-known example is the case of the bacteriophage T4. A single Escherichia coli bacterium injected with a single T4 phage virion at 37 ^oC in rich media lyses after 25-30 minutes, releasing 100-200 phage particles that have replicated themselves inside [4585]. (While lysing is clearly harmful to the bacterium, prior to lysing there is no evidence of purely volumetric-related harm to the microbe.) Taking E. coli volume as 0.6 micron³ (Section 10.4.2.5) and phage T4 volume [4586] as ~200,000 nm³, then the bacteriophage particle load on E. coli at lysis is 3-7% of bacterial cell volume.

In human cells, the tuberculosis bacterium enters the alveolar macrophage which transports the intruder into the blood, the lymphatic system, and elsewhere. Each ~1-micron³ bacillus [4587] that hitches a ride in this manner represents an intrusion of 0.02% of macrophage volume. Other intracellular microorganisms such as Listeria (~0.25 micron³) and Shigella (~2 micron³), once free in the cytoplasm, are propelled “harmlessly” through the cytosol via continuous cytoskeleton-linked actin polymerization (Section 9.4.6). Macrophages infected with Listeria have been observed with ~2% of their volume co-opted by the microbes (~100 organisms) [4588]. While some motile intracellular parasites such as Tyzzer [4589] may cause disarrangement and depopulation of host cell organelles by the movement of their peritrichous flagella, other motile intracellular parasites such as the spotted fever-group Rickettsiae [4590] spread rapidly from cell to cell by actin-based movement but do not cause lysis of the host cell. Typhus-group rickettsiae [4590] multiply in host cells to great numbers without profound damage (until cell lysis finally occurs) – providing a more optimistic biological analog for future medical nanorobots.

Harmful pathogens such as malarial schizonts of Plasmodium falciparum may multiply to 50-70% of erythrocyte cytoplasmic volume before the red cell bursts [4591, 4592]. Other intracellular parasites have been observed at similar cytoplasmic volumetric fractions [4593-4595].

Last updated on 30 April 2004