42
Bioprocess Engineering
or yeast cultures are more often used.
Large protein molecules and antibodies
are made only in mammalian cultures
and product purity is paramount if the
product is an injected biopharmaceuti-
cal. Compounds such as insulin or tissue
plasminogen activator are made in either
system – bacterial or mammalian.
In addition to
E. coli
and CHO cells,
other types of bacterial cells, yeast, plant
cells, and insect cells have been studied for
synthesis of bioproducts. Gram-positive
bacteria such as
Bacillus subtilis
excrete
proteins better but make more proteases
that rapidly degrade the protein products.
The yeast strain,
Saccharomyces cerevisiae
,
is used in the food industry and the
genetics are reasonably well understood;
however, it tends to express only low
levels of a foreign protein and hypergly-
cosylates the product. Plants themselves
offer the advantage of diversity. As much
as 25% of today’s pharmaceuticals (pri-
marily nonprotein products) are extracted
from plants. Plant cell cultures do allow for
more control than using an intact plant;
however, genetic knowledge is less than
what is known for bacterial and animal
cells, and product expression levels are
low. Only a few plant cell systems are used
commercially in Japan and Germany. The
most well known product made in plant
cell culture is the anticancer agent, pacli-
taxel or Taxol. The insect cell–baculovirus
system is used primarily as a research tool
and for small-scale studies (100 L). The
advantages of this system are that it does
allow for high expression of foreign pro-
tein and offers potential safety advantages,
since virus that affect insect cells do not
affect humans. However, the insect sys-
tem does not quite mimic the mammalian
cell system in that the protein product may
have slight structural modiFcations, which
are useful when making vaccines but not
necessarily so for complex products.
Optimization of recombinant protein
production is a continuing objective of in-
dustrial and academic research. Engineer-
ing and science work together to reach this
objective. ±or instance, a common prob-
lem is genetic instability (segregational,
structural, host cell gene mutation, and/or
growth rate dominated), since the over-
production of foreign proteins is always
detrimental to cell growth and survival.
Novel reactor strategies developed to al-
leviate segregational instability include se-
lective recycle reactors in which productive
plasmid-bearing cells are selectively recy-
cled to a fermentor through flocculation
and size separation, while nonproductive
rapidly growing cells and/or dead cells
are removed from the reactor. Thus, a
productive, continuous reactor can exist
‘‘in theory’’ for inFnite time. Cells have
been genetically manipulated to alleviate
this problem as well, by inserting antibi-
otic resistance genes into the plasmids
and supplying antibiotics to the medium.
Research continues to Fnd the ‘‘ideal’’
host–vector system, which rapidly grows
and expresses high levels of foreign pro-
tein that are excreted. The ideal system
also does not produce proteases, and folds
and glycosylates the protein product as
required.
The newest area of bioreactor research
and development is the growth of tissues
in bioFlm or scaffold reactors for regenera-
tion of tissues. Great strides continue to be
made. ±or example, it is now possible for a
patient’s cells to be shipped to a company,
cultured in a scaffold bioreactor and sub-
sequently reimplanted. Regenerative and
rejuvenating therapies may become com-
moninthenea
rfu
tu
re
.Humangene
s
,
proteins, antibodies, and cells are used in
combination to replace, repair, and restore
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